Implement semconv span processor health metrics

[JonasKunz]

Implements the new semantic conventions SDK health metrics for span processors:

• otel.sdk.processor.span.processed for both the SimpleSpanProcessor and BatchingSpanProcessor
• otel.sdk.processor.span.queue.size for BatchingSpanProcessor
• otel.sdk.processor.span.queue.capacity for BatchingSpanProcessor

The metrics are opt-in at the moment and can be enabled via the builders of the processors.
Support for the legacy metrics exposed by the BatchingSpanProcessor is kept. Those are disabled by default too (default MeterProvider is noop) and can be enabled using the same API usage as before (= by providing a MeterProvider).

[codecov]

Codecov Report

Attention: Patch coverage is 89.26554% with 19 lines in your changes missing coverage. Please review.

Project coverage is 89.76%. Comparing base (0ca502b) to head (4b6cf93).

Files with missing lines	Patch %	Lines
...ernal/metrics/LegacyBatchSpanProcessorMetrics.java	89.09%	3 Missing and 3 partials ⚠️
.../internal/metrics/SemConvSpanProcessorMetrics.java	90.62%	2 Missing and 4 partials ⚠️
...k/trace/internal/metrics/SpanProcessorMetrics.java	63.63%	2 Missing and 2 partials ⚠️
...ace/internal/metrics/NoopSpanProcessorMetrics.java	50.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #7441      +/-   ##
============================================
- Coverage     89.77%   89.76%   -0.02%     
- Complexity     6994     7032      +38     
============================================
  Files           798      802       +4     
  Lines         21200    21335     +135     
  Branches       2058     2070      +12     
============================================
+ Hits          19033    19152     +119     
- Misses         1503     1511       +8     
- Partials        664      672       +8     

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[zeitlinger]

looks good - there are merge conflicts though

[JonasKunz]

I'd like to wait with this one until #7430 is approved. Pretty sure there will be review remarks which also apply to this one.

[brunobat]

What if I implement my own exporters? What should I use here?... The same enum values?

[JonasKunz]

First thing you need to decide is if you want your exporter to support the semantic conventions health metrics, e.g. otel.sdk.exporter.span.exported.

If yes, then if you are okay with breaking changes across SDK versions you could use the SemConvExporterMetrics abstraction to record the metrics. Otherwise you'd have to provide your own implementation.

You certainly shouldn't use ExporterType, which are like reserved names for the built-in OTLP exporters.
SemConvExporterMetrics requires a ComponentId, which you can create with a component name of your liking. See the semantic conventions definitions.