Modify MeterProvider to no longer maintain instrument state

[cijothomas]

Changes

Modify MeterProvider to no longer maintain instrument state as the state is expected to come from the API callbacks.
Just a small cleanup, as part of more refactorings in the subsequent PRs.

Please provide a brief description of the changes here.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Design discussion issue #
• Changes in public API reviewed

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (metrics@be1d13f). Click here to learn what that means.
The diff coverage is 77.82%.

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##             metrics    #2134   +/-   ##
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  Coverage           ?   83.59%           
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  Files              ?      211           
  Lines              ?     6692           
  Branches           ?        0           
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  Hits               ?     5594           
  Misses             ?     1098           
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Impacted Files	Coverage Δ
....Exporter.Jaeger/JaegerExporterHelperExtensions.cs	11.11% <0.00%> (ø)
...Telemetry.Exporter.Jaeger/JaegerExporterOptions.cs	100.00% <ø> (ø)
...rc/OpenTelemetry.Exporter.Zipkin/ZipkinExporter.cs	87.64% <ø> (ø)
....Exporter.Zipkin/ZipkinExporterHelperExtensions.cs	11.11% <0.00%> (ø)
...Telemetry.Exporter.Zipkin/ZipkinExporterOptions.cs	100.00% <ø> (ø)
...ion.AspNetCore/AspNetCoreInstrumentationOptions.cs	100.00% <ø> (ø)
src/OpenTelemetry/BaseExportProcessor.cs	84.21% <ø> (ø)
src/OpenTelemetry/BatchLogRecordExportProcessor.cs	0.00% <0.00%> (ø)
src/OpenTelemetry/Metrics/DataPoint/DataPoint.cs	0.00% <0.00%> (ø)
...rc/OpenTelemetry/Metrics/DataPoint/DataPoint{T}.cs	0.00% <0.00%> (ø)
... and 77 more