Metric config modified to look similar to Tracing

[cijothomas]

Building on top of #2034 . Will be iterating on this :)

Changes:
Make configuration (MeterProviderBuilder) similar to Tracing configuration.

using var provider = Sdk.CreateMeterProviderBuilder()
                .AddSource("MeterName") // All instruments from this meter are enabled. Per instrument/more advanced can be added later.
                .SetObservationPeriod(observationInterval)
                .Build();

Added console example.

Next steps:
Add Resource to MeterProvider (similar to Tracing model)
Store the metrics into some inmemory exporter(?), and start a unit test framework.

[codecov]

Codecov Report

Merging #2037 (50e0317) into metrics (c8d8b8c) will increase coverage by 0.00%.
The diff coverage is 89.47%.

@@           Coverage Diff            @@
##           metrics    #2037   +/-   ##
========================================
  Coverage    82.54%   82.54%           
========================================
  Files          205      208    +3     
  Lines         6461     6462    +1     
========================================
+ Hits          5333     5334    +1     
  Misses        1128     1128           

Impacted Files	Coverage Δ
...elemetry/Metrics/MeterProviderBuilderExtensions.cs	66.66% <66.66%> (ø)
...c/OpenTelemetry/Metrics/MeterProviderBuilderSdk.cs	84.61% <80.00%> (-15.39%)	⬇️
src/OpenTelemetry.Api/Metrics/MeterProvider.cs	100.00% <100.00%> (ø)
.../OpenTelemetry.Api/Metrics/MeterProviderBuilder.cs	100.00% <100.00%> (ø)
src/OpenTelemetry/Metrics/MeterProviderSdk.cs	100.00% <100.00%> (ø)
src/OpenTelemetry/Sdk.cs	100.00% <100.00%> (ø)
...stem.Diagnostics.Metrics.Temp/ObservableCounter.cs	0.00% <0.00%> (-80.00%)	⬇️
src/System.Diagnostics.Metrics.Temp/Meter.cs	17.56% <0.00%> (ø)
... and 2 more

[reyang]

Not a blocker for this PR.
I guess we will struggle with the ObservationPeriod since there are scenarios where different instruments being observed and reported at different frequency (e.g. during surgery, temperature is observed and reported every minute, blood pressure is observed and reported every 0.5 second, heart beat is observed every 0.1 second but reported every 15 seconds).

[cijothomas]

agree. Will keep this now to continue experimenting with this.

[reyang]

LGTM as an initial step.

I've left some non-blocking comments that we want to revisit later.