Add initial experimental SDK self-observability metric

[dashpole]

Part of #2547

This adds the OTEL_GO_X_SELF_OBSERVABILITY feature gate, the otel.sdk.span.processor.queue_size metric behind it.

Note: I opted for a single environment variable for all SDKs and for all signals, rather than multiple environment variables.

TODO:

• Add unit tests
• Wait for experimental semantic conventions to be merged

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.2%. Comparing base (5363a52) to head (dda5658).

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #6153     +/-   ##
=======================================
- Coverage   82.3%   82.2%   -0.1%     
=======================================
  Files        273     273             
  Lines      23735   23770     +35     
=======================================
+ Hits       19534   19552     +18     
- Misses      3853    3867     +14     
- Partials     348     351      +3     

see 4 files with indirect coverage changes

[MrAlias]

Looks like a good start, I like the approach. 👍

[dashpole]

Turns out there are already proposed semantic conventions: open-telemetry/semantic-conventions#1631

[dashpole]

I updated this to match the proposed conventions.