Count received batches from conforming clients

[yurishkuro]

Which problem is this PR solving?

• The data loss metrics introduced in [agent] Process data loss stats from clients #2010 cannot be compared with the existing batches.submitted metrics unless all connected clients are emitting the new stats.

Short description of the changes

• Emit a separate client_stats.batches_received counter that only counts batches from "conforming" clients (those that provide clientUUID and batchSeqNo). Then the difference between client_stats.batches_sent and client_stats.batches_received counters reflects the data loss due to UDP packet loss.

[codecov]

Codecov Report

Merging #2030 into master will decrease coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2030      +/-   ##
==========================================
- Coverage   97.41%   97.39%   -0.02%     
==========================================
  Files         207      207              
  Lines       10249    10250       +1     
==========================================
- Hits         9984     9983       -1     
- Misses        221      223       +2     
  Partials       44       44

Impacted Files	Coverage Δ
cmd/agent/app/reporter/client_metrics.go	100% <100%> (ø)	⬆️
cmd/query/app/server.go	91.66% <0%> (-2.78%)	⬇️
plugin/storage/badger/spanstore/reader.go	96.79% <0%> (ø)	⬆️

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