Convert Prometheus Exporter to Pull MetricReader

[lalitb]

Fixes #1941

Changes

Changes are as explained here - #1941 (comment)

Prometheus Exporter now implements MetricReader class, instead of PushMetricExporter. And the metrics is collected only when the actual http pull request is received from Prometheus agent to fetch the metrics. Earlier metrics were fetched periodically by PeriodicMetricReader and stored into intermediate storage, from where the Prometheus agent used to pull the data. This resulted in duplicate entries of data in the intermediate storage.

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

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[codecov]

Codecov Report

Merging #1953 (3bd879d) into main (9200d0c) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1953   +/-   ##
=======================================
  Coverage   87.12%   87.12%           
=======================================
  Files         165      165           
  Lines        4596     4596           
=======================================
  Hits         4004     4004           
  Misses        592      592           

Impacted Files	Coverage Δ
.../include/opentelemetry/sdk/metrics/metric_reader.h	50.00% <ø> (ø)

[lalitb]

This is ready for review now. @esigo - you know this code best, so would need your help in review :)

[esigo]

Thanks for the fix :)
changes look good just had one comment on the unit test