Convert MetricsQueryService to interface

[albertteoh]

Signed-off-by: albertteoh [email protected]

Which problem is this PR solving?

• Relates to "Monitor" tab for service health metrics #2954.

Short description of the changes

• Follow up to suggestion made in this comment.
• Removes much of the unnecessary duplicated proxy code from MetricsQueryService by making it an interface, that embeds the metricsstore.Reader interface.

[codecov]

Codecov Report

Merging #3089 (c865d7d) into master (013efad) will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3089      +/-   ##
==========================================
- Coverage   95.87%   95.83%   -0.04%     
==========================================
  Files         236      235       -1     
  Lines       10073    10067       -6     
==========================================
- Hits         9657     9648       -9     
- Misses        346      347       +1     
- Partials       70       72       +2     

Impacted Files	Coverage Δ
cmd/query/app/server.go	95.62% <ø> (-1.46%)	⬇️
cmd/query/app/static_handler.go	95.16% <0.00%> (-1.62%)	⬇️
plugin/storage/integration/integration.go	77.34% <0.00%> (-0.56%)	⬇️
...lugin/sampling/strategystore/adaptive/processor.go	100.00% <0.00%> (+0.92%)	⬆️

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