Allow configuring the exemplar filter on the metrics SDK

[dashpole]

Part of #5249

Spec

https://opentelemetry.io/docs/specs/otel/metrics/sdk/#exemplarfilter

The ExemplarFilter configuration MUST allow users to select between one of the built-in ExemplarFilters. While ExemplarFilter determines which measurements are eligible for becoming an Exemplar, the ExemplarReservoir makes the final decision if a measurement becomes an exemplar and is stored.

The ExemplarFilter SHOULD be a configuration parameter of a MeterProvider for an SDK. The default value SHOULD be TraceBased. The filter configuration SHOULD follow the environment variable specification.

An OpenTelemetry SDK MUST support the following filters:

• AlwaysOn
• AlwaysOff
• TraceBased

Changes

• adds exemplar.AlwaysOffFilter, which is one of the required filters from the SDK: https://opentelemetry.io/docs/specs/otel/metrics/sdk/#alwaysoff
• adds metric.WithExemplarFilter as an option for the metrics SDK.
• moves handling of OTEL_METRICS_EXEMPLAR_FILTER to the same location as config handling to make code easier to navigate.

dropReservoir can actually be removed, but I plan to do that in a follow-up refactor, since it will be a large diff.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.5%. Comparing base (bc2fe88) to head (a653d53).
Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #5850   +/-   ##
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  Coverage   84.5%   84.5%           
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  Files        272     272           
  Lines      22824   22840   +16     
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+ Hits       19293   19311   +18     
+ Misses      3182    3180    -2     
  Partials     349     349           

see 5 files with indirect coverage changes

[dashpole]

@MrAlias lets wait until after the release to merge. I don't like merging public API changes right before a release.