Initial migration of Prometheus plugin for Elasticsearch to OpenSearch

The main changes are:

- Java package renaming and moving it to where opensearchplugin recommends
- Merging of conflicts (mainly build.gradle)
- Fixing integration tests (testing cluster has different name)
- Configuring correct versions of Gradle (and JDK for GitHub CI)
- Upgrade to Prometheus Java client to the latest version
- Adding missing licenses for dependencies

Closes Aiven-Open#2
Closes Aiven-Open#4

Signed-off-by: lukas-vlcek <lukas.vlcek@aiven.io>

Author: lukas-vlcek

.github/workflows/CI.yml

@@ -0,0 +1,28 @@
+name: Build and Test Plugin Template
+
+on:
+  push:
+    branches:
+      - "*"
+  pull_request:
+    branches:
+      - "*"
+
+jobs:
+  build:
+    strategy:
+      matrix:
+        java: [14]
+    name: Build and Test Plugin Template
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Set up JDK ${{ matrix.java }}
+      uses: actions/setup-java@v1
+      with:
+        java-version: ${{ matrix.java }}
+
+    - name: Build and Run Tests
+      run: |
+          ./gradlew --info build

.github/workflows/ci.yml

@@ -0,0 +1,18 @@
+name: Developer Certificate of Origin Check
+
+on: [pull_request]
+
+jobs:
+  check:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Get PR Commits
+        id: 'get-pr-commits'
+        uses: tim-actions/get-pr-commits@v1.1.0
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}
+      - name: DCO Check
+        uses: tim-actions/dco@v1.1.0
+        with:
+          commits: ${{ steps.get-pr-commits.outputs.commits }}

.github/workflows/dco.yml

@@ -2,22 +2,59 @@
 # https://discuss.elastic.co/t/leftovers-after-integtestrunner-in-root/152610
 \.local*-integTestRunner-execution-times.log
 
-# Eclipse
-.classpath
-.project
-.settings/
-
-# Intellij
+# intellij files
 .idea/
 *.iml
-*.iws
 *.ipr
+*.iws
+build-idea/
+out/
+
+# include shared intellij config
+!.idea/inspectionProfiles/Project_Default.xml
+!.idea/runConfigurations/Debug_OpenSearch.xml
+
+# These files are generated in the main tree by IntelliJ
+benchmarks/src/main/generated/*
+
+# eclipse files
+.project
+.classpath
+.settings
+build-eclipse/
+
+# netbeans files
+nb-configuration.xml
+nbactions.xml
+
+# gradle stuff
+.gradle/
+build/
 
-# Gradle
-.gradle
-build
-gradle-wrapper.jar
-gradle-wrapper.properties
+# vscode stuff
+.vscode/
 
-# Mac
+# testing stuff
+**/.local*
+.vagrant/
+/logs/
+
+# osx stuff
 .DS_Store
+
+# default folders in which the create_bwc_index.py expects to find old es versions in
+/backwards
+/dev-tools/backwards
+
+# needed in case docs build is run...maybe we can configure doc build to generate files under build?
+html_docs
+
+# random old stuff that we should look at the necessity of...
+/tmp/
+eclipse-build
+
+# projects using testfixtures
+testfixtures_shared/
+
+# These are generated from .ci/jobs.t
+.ci/jobs/

.gitignore

@@ -0,0 +1,115 @@
+# Contributing Guidelines
+
+Thank you for your interest in contributing to our project. Whether it's a bug report, new feature, correction, or additional
+documentation, we greatly value feedback and contributions from our community.
+
+Please read through this document before submitting any issues or pull requests to ensure we have all the necessary
+information to effectively respond to your bug report or contribution.
+
+
+## Reporting Bugs/Feature Requests
+
+We welcome you to use the GitHub issue tracker to report bugs or suggest features.
+
+When filing an issue, please check existing open, or recently closed, issues to make sure somebody else hasn't already
+reported the issue. Please try to include as much information as you can. Details like these are incredibly useful:
+
+* A reproducible test case or series of steps
+* The version of our code being used
+* Any modifications you've made relevant to the bug
+* Anything unusual about your environment or deployment
+
+
+## Contributing via Pull Requests
+Contributions via pull requests are much appreciated. Before sending us a pull request, please ensure that:
+
+1. You are working against the latest source on the *main* branch.
+2. You check existing open, and recently merged, pull requests to make sure someone else hasn't addressed the problem already.
+3. You open an issue to discuss any significant work - we would hate for your time to be wasted.
+
+To send us a pull request, please:
+
+1. Fork the repository.
+2. Modify the source; please focus on the specific change you are contributing. If you also reformat all the code, it will be hard for us to focus on your change.
+3. Ensure local tests pass.
+4. Commit to your fork using clear commit messages.
+5. Send us a pull request, answering any default questions in the pull request interface.
+6. Pay attention to any automated CI failures reported in the pull request, and stay involved in the conversation.
+
+GitHub provides additional document on [forking a repository](https://help.github.com/articles/fork-a-repo/) and
+[creating a pull request](https://help.github.com/articles/creating-a-pull-request/).
+
+## Developer Certificate of Origin
+
+OpenSearch is an open source product released under the Apache 2.0 license (see either [the Apache site](https://www.apache.org/licenses/LICENSE-2.0) or the [LICENSE.txt file](LICENSE)). The Apache 2.0 license allows you to freely use, modify, distribute, and sell your own products that include Apache 2.0 licensed software.
+
+We respect intellectual property rights of others and we want to make sure all incoming contributions are correctly attributed and licensed. A Developer Certificate of Origin (DCO) is a lightweight mechanism to do that.
+
+The DCO is a declaration attached to every contribution made by every developer. In the commit message of the contribution, the developer simply adds a `Signed-off-by` statement and thereby agrees to the DCO, which you can find below or at [DeveloperCertificate.org](http://developercertificate.org/).
+
+```
+Developer's Certificate of Origin 1.1
+
+By making a contribution to this project, I certify that:
+
+(a) The contribution was created in whole or in part by me and I
+    have the right to submit it under the open source license
+    indicated in the file; or
+
+(b) The contribution is based upon previous work that, to the
+    best of my knowledge, is covered under an appropriate open
+    source license and I have the right under that license to
+    submit that work with modifications, whether created in whole
+    or in part by me, under the same open source license (unless
+    I am permitted to submit under a different license), as
+    Indicated in the file; or
+
+(c) The contribution was provided directly to me by some other
+    person who certified (a), (b) or (c) and I have not modified
+    it.
+
+(d) I understand and agree that this project and the contribution
+    are public and that a record of the contribution (including
+    all personal information I submit with it, including my
+    sign-off) is maintained indefinitely and may be redistributed
+    consistent with this project or the open source license(s)
+    involved.
+ ```
+
+We require that every contribution to OpenSearch is signed with a Developer Certificate of Origin. Additionally, please use your real name. We do not accept anonymous contributors nor those utilizing pseudonyms.
+
+Each commit must include a DCO which looks like this
+
+```
+Signed-off-by: Jane Smith <jane.smith@email.com>
+```
+
+You may type this line on your own when writing your commit messages. However, if your user.name and user.email are set in your git configs, you can use `-s` or `– – signoff` to add the `Signed-off-by` line to the end of the commit message.
+
+## Finding contributions to work on
+Looking at the existing issues is a great way to find something to contribute on. As our projects, by default, use the default GitHub issue labels (enhancement/bug/duplicate/help wanted/invalid/question/wontfix), looking at any 'help wanted' issues is a great place to start.
+
+
+## Code of Conduct
+This project has adopted the [Amazon Open Source Code of Conduct](https://aws.github.io/code-of-conduct).
+For more information see the [Code of Conduct FAQ](https://aws.github.io/code-of-conduct-faq) or contact
+opensource-codeofconduct@amazon.com with any additional questions or comments.
+
+
+## Security issue notifications
+If you discover a potential security issue in this project we ask that you notify AWS/Amazon Security via our [vulnerability reporting page](http://aws.amazon.com/security/vulnerability-reporting/). Please do **not** create a public github issue.
+
+## License Headers
+
+New files in your code contributions should contain the following license header. If you are modifying existing files with license headers, or including new files that already have license headers, do not remove or modify them without guidance.
+
+```
+/*
+ * Copyright OpenSearch Contributors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+```
+
+## Licensing
+
+See the [LICENSE](LICENSE) file for our project's licensing. We will ask you to confirm the licensing of your contribution.