Remove unnecessary ServiceName index seek if tags query is available

[burmanm]

Which problem is this PR solving?

• Reduce the amount of queries against the badger backend store if Tags are used as a query parameter. Also, if the Tags is the only query, then we also remove single merge-join phase.

Short description of the changes

• Tags' key already includes the ServiceName so it makes no sense to add the ServiceName index seek as we can filter it all in a single index seek.

[codecov]

Codecov Report

Merging #2535 into master will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2535      +/-   ##
==========================================
- Coverage   95.41%   95.38%   -0.04%     
==========================================
  Files         208      208              
  Lines        9219     9222       +3     
==========================================
  Hits         8796     8796              
- Misses        348      350       +2     
- Partials       75       76       +1     

Impacted Files	Coverage Δ
plugin/storage/badger/spanstore/reader.go	95.37% <100.00%> (-0.67%)	⬇️
...lugin/sampling/strategystore/adaptive/processor.go	99.07% <0.00%> (-0.93%)	⬇️
plugin/storage/integration/integration.go	77.90% <0.00%> (+0.55%)	⬆️

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[jpkrohling]

Looks sane to me. Just to confirm: this is covered with test cases already, right?

[burmanm]

Multiple tests have Tags-queries, yes. The TestRandomTraceID fetches a query with Tags & ServiceName parameters and as such tests this code path with just the Tags-query being executed.

[jpkrohling]

Nice to see a PR from you :-)