Allow registering ContextStorage wrappers with code to avoid requirin…

[anuraaga]

…g SPI for listening to events.

@sfriberg reached out in wanting to be able to listen ContextStorage events without overriding the detected storage mechanism, and I think being able to register listeners with code is important in any case. It was the first feature request we got in Armeria after adding the ContextStorage SPI since SPI is too tedious to just do something like pushing MDC.

Fixes #922

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@3e31fd9). Click here to learn what that means.
The diff coverage is 50.00%.

@@            Coverage Diff            @@
##             master    #2044   +/-   ##
=========================================
  Coverage          ?   85.17%           
  Complexity        ?     1982           
=========================================
  Files             ?      228           
  Lines             ?     7827           
  Branches          ?      831           
=========================================
  Hits              ?     6667           
  Misses            ?      848           
  Partials          ?      312           

Impacted Files	Coverage Δ	Complexity Δ
.../java/io/opentelemetry/context/ContextStorage.java	50.00% <0.00%> (ø)	2.00 <0.00> (?)
...ain/java/io/opentelemetry/context/LazyStorage.java	77.77% <50.00%> (ø)	9.00 <0.00> (?)
.../opentelemetry/context/ContextStorageWrappers.java	56.25% <56.25%> (ø)	3.00 <3.00> (?)
...nsion/trace/aws/resource/AwsResourceConstants.java	100.00% <0.00%> (ø)	1.00% <0.00%> (?%)
...java/io/opentelemetry/sdk/trace/data/SpanData.java	100.00% <0.00%> (ø)	0.00% <0.00%> (?%)
.../io/opentelemetry/sdk/trace/TracerSharedState.java	93.33% <0.00%> (ø)	10.00% <0.00%> (?%)
...ain/java/io/opentelemetry/api/baggage/Baggage.java	87.50% <0.00%> (ø)	7.00% <0.00%> (?%)
.../opentelemetry/sdk/metrics/InstrumentRegistry.java	100.00% <0.00%> (ø)	6.00% <0.00%> (?%)
...ntelemetry/sdk/trace/RecordEventsReadableSpan.java	84.28% <0.00%> (ø)	70.00% <0.00%> (?%)
.../opentelemetry/sdk/logging/LogSinkSdkProvider.java	100.00% <0.00%> (ø)	7.00% <0.00%> (?%)
... and 221 more

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[jkwatson]

It probably doesn't matter too much, since this class isn't public, but exposing synchronized methods is generally a bad practice, as it could allow accidental deadlocking if someone synchronizes on this class.

[anuraaga]

Yeah figured it's ok due to the privateness but went with consistency.

[jkwatson]

Looks good to me.

[sfriberg]

Thanks for doing this @anuraaga I think this looks like something that would be easy to use.

Would it make sense to have to be able to register a SPI similar to what is used for the contextstorage and thus instead of manually coding things the ContextStorage init code would scan the class/module path for any classes implementing the ContextStorageWrapper and add those?

[jkwatson]

Thanks for doing this @anuraaga I think this looks like something that would be easy to use.

Would it make sense to have to be able to register a SPI similar to what is used for the contextstorage and thus instead of manually coding things the ContextStorage init code would scan the class/module path for any classes implementing the ContextStorageWrapper and add those?

@sfriberg can you log an issue for this, if you think it's a good idea yourself? Thanks!