Add integration test for ErrTraceNotFound

[yurishkuro]

This is especially interesting when using go-plugin based storage, where the error has to be correctly passed through gRPC client.

The test revealed that Badger did not return NotFound when expected, so it's fixed here as well.

I added documentation to the span Reader API with expectations about not-found.

[objectiser]

LGTM - although the title is a bit misleading, as the integration test was already testing for ErrTraceNotFound, so assume it wasn't being run against badger? But need to fix TestFindNothing before merging.

[jpkrohling]

This is related to #2471, isn't it?

[jpkrohling]

I'm blocking this for now, because of the test failure. Looks like the same is being addressed in another PR already, so, we might want to close this.

[yurishkuro]

although the title is a bit misleading, as the integration test was already testing for ErrTraceNotFound, so assume it wasn't being run against badger

@objectiser where do you see integration test checking for NotFound? Badger implementation clearly did not satisfy the API contract for that (which I also fix in this PR), yet it was passing integration tests, so I assume there was no explicit test for NotFound.

@jpkrohling I don't know if #2471 is fixing the same issue (and it's doing it incorrectly anyway, findTraces can return empty list).

[objectiser]

@yurishkuro yep, lgtm.

[yurishkuro]

I added documentation to the span Reader API with expectations about not-found.

[codecov]

Codecov Report

Merging #2478 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2478      +/-   ##
==========================================
+ Coverage   95.52%   95.53%   +0.01%     
==========================================
  Files         208      208              
  Lines       10750    10756       +6     
==========================================
+ Hits        10269    10276       +7     
  Misses        405      405              
+ Partials       76       75       -1     

Impacted Files	Coverage Δ
plugin/storage/badger/spanstore/reader.go	96.79% <100.00%> (ø)
plugin/storage/integration/integration.go	81.39% <100.00%> (+1.01%)	⬆️
...lugin/sampling/strategystore/adaptive/processor.go	99.20% <0.00%> (-0.80%)	⬇️
cmd/query/app/server.go	90.69% <0.00%> (+1.55%)	⬆️

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[jpkrohling]

Thanks for documenting the interface!