Default to periodic metric reader for push based exporters

[alanwest]

Fixes #2979.

The OTLP exporter was mistakingly defaulted to the manual metric reader.

With this PR:

• The periodic reader is reestablished as the default for the OTLP exporter with a default interval of 60 seconds.
• The Console and InMemory exporters now default to using the periodic reader, but with an infinite interval by default (i.e., effectively the same as manual). There will be a spec PR to follow to define what the default interval should be for Console and InMemory exporters.
• As a consequence, the PeriodicExportingMetricReader now allows an infinite export interval.

[codecov]

Codecov Report

Merging #2982 (fde279f) into main (5f509bf) will increase coverage by 0.59%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #2982      +/-   ##
==========================================
+ Coverage   84.13%   84.72%   +0.59%     
==========================================
  Files         258      258              
  Lines        9093     9095       +2     
==========================================
+ Hits         7650     7706      +56     
+ Misses       1443     1389      -54     

Impacted Files	Coverage Δ
...rter.InMemory/InMemoryExporterMetricsExtensions.cs	68.00% <100.00%> (+4.00%)	⬆️
...tryProtocol/Implementation/MetricItemExtensions.cs	93.79% <100.00%> (+0.26%)	⬆️
...nTelemetryProtocol/OtlpMetricExporterExtensions.cs	71.42% <100.00%> (+7.14%)	⬆️
src/OpenTelemetry/Metrics/MetricReaderOptions.cs	100.00% <100.00%> (+25.00%)	⬆️
...Telemetry/Metrics/PeriodicExportingMetricReader.cs	76.47% <100.00%> (+76.47%)	⬆️
...ry/Metrics/PeriodicExportingMetricReaderOptions.cs	100.00% <100.00%> (ø)
...lemetry/Internal/SelfDiagnosticsConfigRefresher.cs	86.53% <0.00%> (-2.89%)	⬇️
...nTelemetry/Internal/OpenTelemetrySdkEventSource.cs	73.68% <0.00%> (-1.76%)	⬇️
...OpenTelemetry/Metrics/BaseExportingMetricReader.cs	86.95% <0.00%> (+1.44%)	⬆️
...emetry.Api/Internal/OpenTelemetryApiEventSource.cs	82.35% <0.00%> (+2.94%)	⬆️
... and 6 more

[tomkerkhove]

Idea - Might be good to have the default outside of this extension in a constant somewhere; but I believe it is already defined somewhere else so using that is OK as well

[alanwest]

Moved to a constant. The constant already defined is part of the SDK project and is internal so not accessible here.

[alanwest]

Actually come to think of it the right thing to do here is to just do new PeriodicExportingMetricReader(metricExporter);. I've got a follow up PR to remove MetricReaderType. I'll clean that up in that PR.