Add Meter and MeterProvider in the SDK

[lalitb]

Add

• Meter and MeterProvider in the SDK,
• placeholder for MeterExporter, MeterReader and View class.
• Internally used MeterContext class.

[codecov]

Codecov Report

Merging #1078 (f6df863) into main (6b9ed3d) will decrease coverage by 1.02%.
The diff coverage is 51.32%.

@@            Coverage Diff             @@
##             main    #1078      +/-   ##
==========================================
- Coverage   94.41%   93.39%   -1.01%     
==========================================
  Files         158      165       +7     
  Lines        6077     6229     +152     
==========================================
+ Hits         5737     5817      +80     
- Misses        340      412      +72     

Impacted Files	Coverage Δ
api/include/opentelemetry/metrics/meter.h	100.00% <ø> (ø)
...lude/opentelemetry/sdk/common/global_log_handler.h	80.00% <ø> (+8.00%)	⬆️
sdk/src/metrics/meter.cc	14.29% <14.29%> (ø)
sdk/src/metrics/meter_context.cc	40.00% <40.00%> (ø)
sdk/src/metrics/meter_provider.cc	78.13% <78.13%> (ø)
sdk/test/metrics/meter_provider_sdk_test.cc	80.00% <80.00%> (ø)
...nclude/opentelemetry/sdk/metrics/metric_exporter.h	100.00% <100.00%> (ø)
.../include/opentelemetry/sdk/metrics/metric_reader.h	100.00% <100.00%> (ø)
sdk/include/opentelemetry/sdk/metrics/view.h	100.00% <100.00%> (ø)