Mention OTEL_METRIC_EXPORT_INTERVAL in Java manual instrumentation page. (#3786)

Author: eplusx

content/en/docs/languages/java/getting-started.md

@@ -144,11 +144,18 @@ number of ways, the steps below use environment variables.
    export JAVA_TOOL_OPTIONS="-javaagent:PATH/TO/opentelemetry-javaagent.jar" \
      OTEL_TRACES_EXPORTER=logging \
      OTEL_METRICS_EXPORTER=logging \
-     OTEL_LOGS_EXPORTER=logging
+     OTEL_LOGS_EXPORTER=logging \
+     OTEL_METRIC_EXPORT_INTERVAL=15000
    ```
 
-   {{% alert title="Important" color="warning" %}}Replace `PATH/TO` above, with
-   your path to the JAR.{{% /alert %}}
+   {{% alert title="Important" color="warning" %}}
+
+   - Replace `PATH/TO` above, with your path to the JAR.
+   - Set `OTEL_METRIC_EXPORT_INTERVAL` to a value well below the default, as we
+     illustrate above, **only during testing** to help you more quickly ensure
+     that metrics are properly generated.
+
+   {{% /alert %}}
 
 3. Run your **application** once again:
 

content/en/docs/languages/java/instrumentation.md

@@ -419,12 +419,17 @@ OTEL_SERVICE_NAME=dice-server \
 OTEL_TRACES_EXPORTER=logging \
 OTEL_METRICS_EXPORTER=logging \
 OTEL_LOGS_EXPORTER=logging \
+OTEL_METRIC_EXPORT_INTERVAL=15000 \
 java -jar ./build/libs/java-simple.jar
 ```
 
 This basic setup has no effect on your app yet. You need to add code for
 [traces](#traces), [metrics](#metrics), and/or [logs](#logs).
 
+Note that `OTEL_METRIC_EXPORT_INTERVAL=15000` (milliseconds) is a temporary
+setting to test that your metrics are properly generated. Remember to remove the
+setting once you are done testing. The default is 60000 milliseconds.
+
 #### Manual Configuration
 
 `OpenTelemetrySdk.builder()` returns an instance of `OpenTelemetrySdkBuilder`,
@@ -1547,6 +1552,11 @@ example `OTEL_TRACES_EXPORTER=jaeger` configures your application to use the
 Jaeger exporter. The corresponding Jaeger exporter library has to be provided in
 the classpath of the application as well.
 
+If you use the `console` or `logging` exporter for metrics, consider temporarily
+setting `OTEL_METRIC_EXPORT_INTERVAL` to a small value like `15000`
+(milliseconds) while testing that your metrics are properly recorded. Remember
+to remove the setting once you are done testing.
+
 It's also possible to set up the propagators via the `OTEL_PROPAGATORS`
 environment variable, like for example using the `tracecontext` value to use
 [W3C Trace Context](https://www.w3.org/TR/trace-context/).