Enable metric collection from MetricReader

[lalitb]

Fixes #1091

Changes

This PR enables MetricReader to invoke metric collection. The flow is drawn (with red) in block diagram.

1. MetricReader : Acts as an interface to start the metric collection.
2. MetricCollector - This is an internal class to perform actual metric collection, and abstracts MetricReader from any implementation changes.

Flow

MetricReader::Collect() -> MetricCollector::Collect() -> Meter::Collect() -> AggregationStorage::Collect()

1. MetricReader::Collect(): Invokes MetricCollector::Collect()
2. MetricCollector::Collect() - Fetches all the configured Meters from MeterContext, and invoke Meter::Collect() on each of them.
3. Meter::Collect() - Fetches the configured Aggregation storages for each created instrument ( based on View configuration ), and invoke Collect() on them.

MetricReader and CollectorHandle are base classes for MetricCollector and are meant to be opaque handles over MetricCollector.

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

[codecov]

Codecov Report

Merging #1241 (1b6af65) into main (6bd2f96) will decrease coverage by 0.39%.
The diff coverage is 41.06%.

@@            Coverage Diff             @@
##             main    #1241      +/-   ##
==========================================
- Coverage   92.37%   91.99%   -0.38%     
==========================================
  Files         203      205       +2     
  Lines        7338     7395      +57     
==========================================
+ Hits         6778     6802      +24     
- Misses        560      593      +33     

Impacted Files	Coverage Δ
...nclude/opentelemetry/sdk/metrics/metric_exporter.h	0.00% <ø> (-50.00%)	⬇️
...e/opentelemetry/sdk/metrics/state/metric_storage.h	100.00% <ø> (ø)
...ntelemetry/sdk/metrics/state/sync_metric_storage.h	67.75% <0.00%> (ø)
...clude/opentelemetry/sdk/metrics/sync_instruments.h	100.00% <ø> (ø)
sdk/src/metrics/meter.cc	7.90% <0.00%> (-0.92%)	⬇️
sdk/src/metrics/sync_instruments.cc	100.00% <ø> (ø)
sdk/src/metrics/metric_reader.cc	22.86% <22.86%> (ø)
sdk/test/metrics/meter_provider_sdk_test.cc	75.68% <25.00%> (-2.70%)	⬇️
sdk/src/metrics/meter_context.cc	37.50% <27.28%> (+1.14%)	⬆️
.../include/opentelemetry/sdk/metrics/metric_reader.h	50.00% <50.00%> (-50.00%)	⬇️
... and 10 more

[esigo]

LGTM
Thanks for the PR