Add config to enable Default Exponential Histogram for Prometheus Exporter

[Abhishekkr3003]

This PR adds support for config otel.exporter.prometheus.metrics.default.histogram.aggregation by which we can make BASE2_EXPONENTIAL_BUCKET_HISTOGRAM as the default histogram aggregation for Prometheus exporter.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.31%. Comparing base (38aefff) to head (9cba997).
Report is 16 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #6541      +/-   ##
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- Coverage     90.59%   90.31%   -0.28%     
- Complexity     6255     6265      +10     
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  Files           689      690       +1     
  Lines         18698    18776      +78     
  Branches       1843     1854      +11     
============================================
+ Hits          16939    16958      +19     
- Misses         1201     1258      +57     
- Partials        558      560       +2     

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[jkwatson]

Hello, @Abhishekkr3003 . The environment variable/configuration option you're introducing here doesn't appear to be one of the official configuration options (see here for the full list of official env vars): https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/configuration/sdk-environment-variables.md

In addition, I believe there is a moratorium currently on introducing new environment variables until the configuration specification is complete.

If we are going to introduce a new environment variable here, we would absolutely have to make it an "experimental" env var, and have the word "experimental" in the name of the config option. See here for an example: https://github.com/open-telemetry/opentelemetry-java/blob/main/exporters/common/src/main/java/io/opentelemetry/exporter/internal/ExporterBuilderUtil.java#L44

However, I would strongly recommend that you bring this up at the next Java SIG meeting to see if there is an appetite to add new configuration options like this.

[jack-berg]

@Abhishekkr3003 I added a test which confirms that base2 exponential histograms are in fact configured by requesting the protobuf binary format from the prometheus server, parsing it to equivalent java bindings, and asserting against its string represetnation.

[Abhishekkr3003]

Thanks, @jack-berg , for continuing it, I have not been getting time to contribute here lately.