[otlp] Update README for UseOtlpExporter & new experimental flag

[CodeBlanch]

Relates to #5400
Relates to #5435

Changes

• OtlpExporter README updates for [otlp] UseOtlpExporter cross-cutting extension #5400 & [otlp] Add experimental feature flag for enabling retries #5435

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.46%. Comparing base (6250307) to head (8e2e25a).
Report is 136 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5452      +/-   ##
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+ Coverage   83.38%   85.46%   +2.08%     
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  Files         297      289       -8     
  Lines       12531    12463      -68     
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+ Hits        10449    10652     +203     
+ Misses       2082     1811     -271     

Flag	Coverage Δ
unittests	?
unittests-Solution-Experimental	85.25% <100.00%> (?)
unittests-Solution-Stable	85.40% <100.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
...lder/OpenTelemetryBuilderOtlpExporterExtensions.cs	100.00% <100.00%> (ø)

... and 63 files with indirect coverage changes

[utpilla]

This sounds a little confusing. We start off saying calls to the "With" extension methods are not required and then go on to say however only the telemetry enabled using "With" extension methods would be exported.

Maybe update this to something like:

When calling UseOtlpExporter method, you don't need to separately call AddOtlpExporter method for WithLogging, WithTracing, and/or WithMetrics.

[CodeBlanch]

This is kind of tricky. The docs we have that I could find seem to all show "Sdk.Create*" style which really doesn't help for this cross-cutting extension using the hosting style. I decided to show some real examples of how to do it. LMK what you think.

[vishweshbankwar]

WithLogging - This is not a stable way of configuring logging today so I think we should not mention it.

To keep things simple, what do you guys think about something like this.

Using UseOtlpExporter automatically enables the OTLP exporter for logs, metrics, and traces. 
For additional sdk configuration, like adding meters to the MeterProvider, 
incorporating sources to the TracerProvider, or 
configuring resources for all three types of signals, additional setup will required. 
For more information, refer to the details below. (Followed by what we have below)

[CodeBlanch]

I removed the mention of the "With" calls and simplified it down to just...

Calling UseOtlpExporter automatically enables logging, metrics, and tracing however only telemetry which has been enabled will be exported.

[utpilla]

Left some minor comments. LGTM overall.

[reyang]

@vishweshbankwar I have questions regarding the name - does it mean later there will be something like "OTLP_ENABLE_ONDISK_RETRY? What's the thinking which leads to the current name instead of OTEL_DOTNET_EXPERIMENTAL_OTLP_ENABLE_RETRY`?

[vishweshbankwar]

does it mean later there will be something like "OTLP_ENABLE_ONDISK_RETRY

That was the thinking. Wanted to have decoupled experimental environment variables.

When we move to stable, I would like it to be OTEL_DOTNET_OTLP_ENABLE_RETRY and define possible values for it like inmemory and disk.

[reyang]

LGTM with couple non-blocking comments.