Add metrics exporter modes

[reyang]

Introduced ExportModes attribute for metric exporters.
With the change, the SDK will throw exception if the user is trying to setup a PeriodicExportingMetricReader with an exporter which only supports pull mode.

PrometheusExporter will look like:

    [AggregationTemporality(AggregationTemporality.Cumulative)]
    [ExportModes(ExportModes.Pull)]
    public class PrometheusExporter : BaseExporter<Metric>
    {
        ...
    }

If someone is doing this:

provider.AddMetricReader(new PeriodicExportingMetricReader(new PrometheusExporter(...)))

an InvalidOperationException("The exporter does not support push mode.") will be thrown.

[codecov]

Codecov Report

Merging #2353 (5860327) into main (2df1a62) will decrease coverage by 0.45%.
The diff coverage is 27.77%.

@@            Coverage Diff             @@
##             main    #2353      +/-   ##
==========================================
- Coverage   81.08%   80.62%   -0.46%     
==========================================
  Files         228      229       +1     
  Lines        7305     7318      +13     
==========================================
- Hits         5923     5900      -23     
- Misses       1382     1418      +36     

Impacted Files	Coverage Δ
src/OpenTelemetry/Metrics/ExportModesAttribute.cs	0.00% <0.00%> (ø)
...Telemetry/Metrics/PeriodicExportingMetricReader.cs	0.00% <0.00%> (ø)
...OpenTelemetry/Metrics/BaseExportingMetricReader.cs	71.42% <45.45%> (-4.77%)	⬇️
...ation.AspNet.TelemetryHttpModule/ActivityHelper.cs	56.14% <0.00%> (-28.08%)	⬇️
src/OpenTelemetry.Api/Context/RuntimeContext.cs	63.15% <0.00%> (-13.16%)	⬇️
...emetry.Api/Context/AsyncLocalRuntimeContextSlot.cs	87.50% <0.00%> (-12.50%)	⬇️
...Zipkin/Implementation/ZipkinExporterEventSource.cs	63.63% <0.00%> (-9.10%)	⬇️
....TelemetryHttpModule/AspNetTelemetryEventSource.cs	23.33% <0.00%> (-6.67%)	⬇️
...ZPages/Implementation/ZPagesExporterEventSource.cs	56.25% <0.00%> (-6.25%)	⬇️
...Telemetry/Internal/SelfDiagnosticsEventListener.cs	96.85% <0.00%> (-0.79%)	⬇️
... and 1 more