Remove existing Metrics code

[cijothomas]

Given the fact that the whole spec for metrics is re-written, it makes sense to rebuild the implementation.

Removes API, SDK and Prometheus Exporter. Will use git history to get back still useful code especially from Prometheus exporter (when we are ready to write prometheus exporter for new sdk)

[reyang]

LGTM - probably need to update the changelog?

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (metrics@b1df9db). Click here to learn what that means.
The diff coverage is 81.56%.

@@            Coverage Diff             @@
##             metrics    #2030   +/-   ##
==========================================
  Coverage           ?   83.89%           
==========================================
  Files              ?      192           
  Lines              ?     6184           
  Branches           ?        0           
==========================================
  Hits               ?     5188           
  Misses             ?      996           
  Partials           ?        0           

Impacted Files	Coverage Δ
...metry.Api/Context/Propagation/BaggagePropagator.cs	83.92% <ø> (ø)
....Exporter.Jaeger/JaegerExporterHelperExtensions.cs	11.11% <0.00%> (ø)
....Exporter.Zipkin/ZipkinExporterHelperExtensions.cs	11.11% <0.00%> (ø)
src/OpenTelemetry/ProviderExtensions.cs	50.00% <0.00%> (ø)
src/OpenTelemetry/Sdk.cs	100.00% <ø> (ø)
src/OpenTelemetry/Trace/SamplingParameters.cs	76.92% <ø> (ø)
...s.Hosting/Trace/TracerProviderBuilderExtensions.cs	17.64% <17.64%> (ø)
...metryProtocol/OtlpTraceExporterHelperExtensions.cs	72.22% <33.33%> (ø)
...ing/Implementation/TracerProviderBuilderHosting.cs	38.46% <38.46%> (ø)
...rc/OpenTelemetry/Trace/TracerProviderExtensions.cs	32.25% <45.45%> (ø)
... and 42 more