fix: clamp negative count metric values to 0

[valyria257]

Proposed changes

If nginx reset the values of the counters used for these metric values,
then the gauge values will be negative because they are calculated
based on the current value minus the previous value.

Since these gauges are tracking counts, it doesn't make sense for them
to be negative. The current value should always be greater than the
previous because it is based on a counter, unless it's been reset to 0.

Thus, this will set the gauge values to 0 if the delta is negative.

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• I have run make install-tools and have attached any dependency changes to this pull request
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• If applicable, I have updated any relevant documentation (README.md)
• If applicable, I have tested my cross-platform changes on Ubuntu 22, Redhat 8, SUSE 15 and FreeBSD 13

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.00%. Comparing base (59d9c4d) to head (377b4f3).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1690      +/-   ##
==========================================
+ Coverage   84.99%   85.00%   +0.01%     
==========================================
  Files         104      104              
  Lines       13579    13579              
==========================================
+ Hits        11541    11543       +2     
+ Misses       1524     1522       -2     
  Partials      514      514              

Files with missing lines	Coverage Δ
...collector/nginxplusreceiver/record/http_metrics.go	100.00% <100.00%> (ø)
.../nginxplusreceiver/record/location_zone_metrics.go	100.00% <100.00%> (ø)
...or/nginxplusreceiver/record/server_zone_metrics.go	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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