Create Grocery Example for Metrics API

[victlu]

Create Grocery example per Metrics API SIG.

[codecov]

Codecov Report

Merging #1831 (6e6ff21) into metrics (c8d8b8c) will increase coverage by 1.36%.
The diff coverage is n/a.

❗ Current head 6e6ff21 differs from pull request most recent head 7e7306b. Consider uploading reports for the commit 7e7306b to get more accurate results

@@             Coverage Diff             @@
##           metrics    #1831      +/-   ##
===========================================
+ Coverage    82.54%   83.91%   +1.36%     
===========================================
  Files          205      192      -13     
  Lines         6461     6184     -277     
===========================================
- Hits          5333     5189     -144     
+ Misses        1128      995     -133     

Impacted Files	Coverage Δ
src/System.Diagnostics.Metrics.Temp/Measurement.cs
src/OpenTelemetry/Metrics/MeterProvider.cs
src/System.Diagnostics.Metrics.Temp/Counter.cs
...m.Diagnostics.Metrics.Temp/ObservableInstrument.cs
...iagnostics.Metrics.Temp/ObservableUpDownCounter.cs
src/System.Diagnostics.Metrics.Temp/InstrumentT.cs
...System.Diagnostics.Metrics.Temp/ObservableGauge.cs
...stem.Diagnostics.Metrics.Temp/ObservableCounter.cs
...c/OpenTelemetry/Metrics/MeterProviderBuilderSdk.cs
...c/System.Diagnostics.Metrics.Temp/MeterListener.cs
... and 3 more

[victlu]

Ping to see if we should approve this?

[reyang]

Ping to see if we should approve this?

I cannot approve this PR. I think the goal is to experiment + discuss this during the metrics API/SDK design, rather than ship code to the metrics branch at this phase.