Add specs for empty telemetry envelopes.

[djspinmonkey]

Based on the discussion here, this PR adds some guidelines for handling empty telemetry envelopes. (Specifically, that they should generally be avoided and may be dropped.)

This is my first PR to the OTel spec, so please let me know if there's a better place to put this or I missed any steps of the process!

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: djspinmonkey / name: John Hyland (dab9f5d, cca6f46, bb8620e)

[cijothomas]

• ❌ - login: @djspinmonkey / name: John Hyland . The commit (cca6f46) is not authorized under a signed CLA. Please click here to be authorized. For further assistance with EasyCLA, please submit a support request ticket.

@djspinmonkey A mandatory pre-requisite for contributions to OTel is to sign the EasyCLA. Could you follow the link and make sure it is done.. (If contributing on behalf of an employer, you may want to consult them too)

https://github.com/open-telemetry/community/blob/main/guides/contributor/CLA.md

[djspinmonkey]

Thanks, I just filled that out! :-)

[tigrannajaryan]

I wonder if a better place for this comment is in proto files, e.g. as a comment of the mesage:

opentelemetry-proto/opentelemetry/proto/metrics/v1/metrics.proto

Line 66 in 6a4f4e4

message ResourceMetrics {

[djspinmonkey]

In terms of the placement, I'm not sure about putting this in the metrics proto, since it's not specific to metrics - it would apply equally to logs or traces. It also doesn't really feel like a detail of the protobuff format to me, since it mostly applies (at least in my use case) to resources being processed that have already been hydrated into in-memory structs, rather than anything about the line format.

That said, of course if that's where you think it should go, I'm happy to move it. Would you want it copied out into all three of metrics, logs, and traces protos if so?

[tigrannajaryan]

In terms of the placement, I'm not sure about putting this in the metrics proto, since it's not specific to metrics - it would apply equally to logs or traces.

What we do currently is just copy/paste the same comment to all signals. Not great, I agree, but that's the current practice.

I don't have a strong opinion on the placement. Let's see others think.

[djspinmonkey]

Just following up here, are there any other changes folks would like to see before merging? I can copy it out into the three protobuff files, if that's the preference.