[docs] Adjust filtering processor guidance

[CodeBlanch]

The previous "FilteringProcessor" example required manually constructing the exporter and passing it into the processor. Many exporters do not have a public ctor so it is not possible to construct them.

Changes

Switched the example to toggle Activity.Recorded instead. This works because BatchActivityExportProcessor and SimpleActivityExportProcessor both no-op for this case.

[codecov]

Codecov Report

Merging #3834 (b90b4fa) into main (d954c5b) will decrease coverage by 0.08%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3834      +/-   ##
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- Coverage   87.48%   87.40%   -0.09%     
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  Files         280      280              
  Lines       10767    10767              
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- Hits         9420     9411       -9     
- Misses       1347     1356       +9     

Impacted Files	Coverage Δ
...Propagators/OpenTelemetryPropagatorsEventSource.cs	87.50% <0.00%> (-12.50%)	⬇️
...emetry.Api/Internal/OpenTelemetryApiEventSource.cs	76.47% <0.00%> (-5.89%)	⬇️
...lementation/SqlClientInstrumentationEventSource.cs	70.83% <0.00%> (-4.17%)	⬇️
...lemetry/Internal/SelfDiagnosticsConfigRefresher.cs	86.53% <0.00%> (-2.89%)	⬇️
...nTelemetry/Internal/OpenTelemetrySdkEventSource.cs	80.15% <0.00%> (-2.39%)	⬇️
...ter.ZPages/Implementation/ZPagesActivityTracker.cs	100.00% <0.00%> (+2.85%)	⬆️

[vishweshbankwar]

@CodeBlanch - Filter by setting activity.IsAllDataRequested = false during OnStart. Should we document that as well?. (That will prevent all exporters to export it).

[CodeBlanch]

@vishweshbankwar Customer wants to filter based on some data on the Activity. Duration for example. OnEnd is really the most general-purpose spot we have to filter where all the data will be available. We could also have an OnStart example which uses IsAllDataRequested but more to document there to explain the limited dataset. My concern would be having both might be confusing. @cijothomas How you feel about it?

[cijothomas]

We should be only showing the OnEnd based filter as shown in this PR. There is not much value in showing Filter based on OnStart(), as Sampler is a better fit there. OnEnd is more like "tail" based filtering, based on data not available at SpanStart/Sample time.

[cijothomas]

LGTM