Add grpc.received.message_count and grpc.sent.message_count

[luke-sterkowicz]

Propose new conventions

I propose to add 2 new span attributes for gRPC: grpc.received.message_count and grpc.sent.message_count. The purpose of the attributes is streaming of messages within a single gRPC transaction, which can be configured for request and/or response separately. The context of the attributes would depend on whether the collection is tracked in client or server:

1. Client context

• grpc.sent.message_count the number of request messages sent to server
• grpc.received.message_count the number of response messages received from server

2. Server context

• grpc.received.message_count the number of request messages received from client
• grpc.sent.message_count the number of response messages sent to client

In case of regular (non-streamed) request and/or response, the count would always be 1 (unless an error occurs before or during the transfer).

[mx-psi]

IMO we should look into #1773 first and see if there are metrics that cover those use cases, and adopt those instead. We may not need to solve the whole "aligning both projects" problems, but we should at least not worsen it