Hosting: Support metrics registration via IServiceCollection & deferred configuration

[CodeBlanch]

Changes

• Adds AddOpenTelemetryMetrics extensions on IServiceCollection
• Renames AddMetricReader -> AddReader. This seemed more consistent with the other methods (AddProcessor, SetSampler, AddInstrumentation, etc.)
• Adds IDeferredMeterProviderBuilder to support DI scenarios

Details

Follows closely the same patterns used for TracerProviderBuilder + extensions.

TODOs

• CHANGELOG.md updated for non-trivial changes
• Changes in public API reviewed

[reyang]

LGTM.

[pellared]

nit: Maybe it would be better to add this null guard to public methods?

[CodeBlanch]

I'm going to vote to leave as-is on this one. Reasoning: services is the thing being extended. It is unlikely to be null unless someone is going out of their way to call it directly AND pass null. So given that should be rare, having it here saves having to duplicate the check in all the publics. 2 checks instead of 4, basically.

[codecov]

Codecov Report

Merging #2412 (5f50191) into main (8f284e0) will decrease coverage by 0.04%.
The diff coverage is 77.57%.

❗ Current head 5f50191 differs from pull request most recent head 0643496. Consider uploading reports for the commit 0643496 to get more accurate results

@@            Coverage Diff             @@
##             main    #2412      +/-   ##
==========================================
- Coverage   80.27%   80.23%   -0.05%     
==========================================
  Files         232      235       +3     
  Lines        7505     7564      +59     
==========================================
+ Hits         6025     6069      +44     
- Misses       1480     1495      +15     

Impacted Files	Coverage Δ
...rter.InMemory/InMemoryExporterMetricsExtensions.cs	0.00% <0.00%> (ø)
...nTelemetryProtocol/OtlpMetricExporterExtensions.cs	0.00% <0.00%> (ø)
...Telemetry/Trace/TracerProviderBuilderExtensions.cs	88.23% <0.00%> (ø)
...ensions.Hosting/OpenTelemetryServicesExtensions.cs	64.70% <64.70%> (ø)
....Hosting/Metrics/MeterProviderBuilderExtensions.cs	68.42% <68.42%> (ø)
...s.Hosting/Implementation/TelemetryHostedService.cs	69.23% <75.00%> (-0.77%)	⬇️
...ing/Implementation/TracerProviderBuilderHosting.cs	85.71% <75.00%> (-6.60%)	⬇️
...ting/Implementation/MeterProviderBuilderHosting.cs	85.71% <85.71%> (ø)
...elemetry/Metrics/MeterProviderBuilderExtensions.cs	72.72% <85.71%> (-5.06%)	⬇️
.../OpenTelemetry/Metrics/MeterProviderBuilderBase.cs	88.88% <88.88%> (ø)
... and 9 more

[cijothomas]

@CodeBlanch
Since https://github.com/open-telemetry/opentelemetry-dotnet/pull/2411/files is merged, this PR requires some updates to remove unused using statements else CI will fail.