Make it possible to indicate partial success in an OTLP export response [2]

[joaopgrassi]

This PR is a second attempt in solving/addressing open-telemetry/opentelemetry-specification#2454. This uses the rejected semantics instead of accepted as discussed in the initial PR #390.

Related spec PR: open-telemetry/opentelemetry-specification#2696

The need for this new PR was that with accepted we were running into problems with the 0 value being the same as the default/not set, plus general intention to avoid having optional on the fields.

I did some tests with sample apps to confirm the cases for backwards compatibility. Those can be checked in: joaopgrassi/otlp-partial-success-experimental. The TL;DR: Since it's a new field no breaking change is introduced. New sender will always have to check if the partial_success is set because they can be working with servers using the old protos.

Some context on why this feature is desired:

This feature IS intended to help in the following scenarios:

• Enable servers to provide information to users about a partial success without having to return a 400 - BadRequest (status quo today)
• Enable senders to generate metrics about themselves (otlp.exporter.rejected.data_points=x)
• Enable senders to log messages when a partial success is encountered (very useful to troubleshoot things quickly, without having to reach/search for your vendor's logs for ex)
• Capture situations that are not necessarily client or server errors.
  • Change the type of metric instrument after an initial submission - Gauge > Counter
  • Customers reaching their quotas in the middle of an export request
  • Data not conforming with vendor's constraints - metric keys with not accepted characters coming from a library outside the application's owner control (with the errors they can rename them with a view for ex)
  • OTLP data arriving with timestamp in the past - no explicit contract for this in OTel. Might be tricky for users to debug what's going on

This feature IS NOT intended to help in the following scenarios:

• Enable clients to retry a partial success request
  • Complicated to handle. Needs more structure, semantic convention on typical errors. Was discussed in the initial proto PR
  • This can always be added later on in a non-breaking manner. I started a separated discussion on this topic for metrics here
• Identify which exact log line/data point/span was rejected. Similar to the point above, it needs more discussion on how this can be done and which structure we should have. Always can be done/improved later

cc @tigrannajaryan @jmacd @jsuereth @jack-berg @reyang

[jmacd]

Great, thank you!