[agent] Do not increment data loss counters on the first client batch

[yurishkuro]

Which problem is this PR solving?

In #2010, the counters are always incremented by the delta from current client stats and the previously recorded values. However, when previous values are zero, as could be the case when the client has been running for a while but agent was just restarted, the delta could be very large and not reflective of the incremental change since the previous batch (which the agent did not see), causing massive spikes in metrics and messing up the totals.

Short description of the changes

• Stats from the first batch are captured, but not applied as deltas to the counters.

[codecov]

Codecov Report

Merging #2028 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2028      +/-   ##
==========================================
+ Coverage   97.41%   97.41%   +<.01%     
==========================================
  Files         207      207              
  Lines       10247    10249       +2     
==========================================
+ Hits         9982     9984       +2     
  Misses        222      222              
  Partials       43       43

Impacted Files	Coverage Δ
cmd/agent/app/reporter/client_metrics.go	100% <100%> (ø)	⬆️

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