Trace sdk processor metrics

[dashpole]

trafficstars

Adds otel.sdk.processor.span.queue.size, otel.sdk.processor.span.queue.capacity, and otel.sdk.processor.span.processed.count metrics to the trace batch span processor.

These are defined in https://github.com/open-telemetry/semantic-conventions/blob/cb11bb9bac24f4b0e95ad0f61ce01813d8ceada8/docs/otel/sdk-metrics.md, and are experimental. Because of this, metrics are behind the OTEL_GO_X_SELF_OBSERVABILITY feature gate.

Given the feature is experimental, it always uses the global meterprovider when enabled.

[codecov[bot]]

Codecov Report

:x: Patch coverage is 80.64516% with 12 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 83.0%. Comparing base (c5e68b2) to head (e162018). :warning: Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
sdk/trace/batch_span_processor.go	80.3%	8 Missing and 4 partials :warning:

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #6393   +/-   ##
=====================================
  Coverage   82.9%   83.0%           
=====================================
  Files        264     264           
  Lines      24574   24634   +60     
=====================================
+ Hits       20392   20451   +59     
+ Misses      3802    3800    -2     
- Partials     380     383    +3     

Files with missing lines	Coverage Δ
sdk/trace/provider.go	86.4% <ø> (+1.0%)	:arrow_up:
sdk/trace/tracer.go	95.9% <100.0%> (ø)
sdk/trace/batch_span_processor.go	80.4% <80.3%> (+2.1%)	:arrow_up:

... and 2 files with indirect coverage changes

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[dashpole]

I remember why I didn't get this merged now... Unit tests for the queue size are hard to write without race conditions.

[dashpole]

Figured out how to get unit tests to work.

[dashpole]

@pellared rebased on your PR, and updated to use your feature gate

[dashpole]

Codecov failure is from not testing otel.Handle(err) calls.