Fix bug where an empty exemplar was added to counters

[dashpole]

Fixes #5383

I also found out that the +Inf bucket is only sometimes added to the proto format: https://github.com/prometheus/client_golang/blob/d038ab96c0c7b9cd217a39072febd610bcdf1fd8/prometheus/metric.go#L189

That meant we were dropping the highest histogram bucket if users were not using exemplars.

@gouthamve

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 62.1%. Comparing base (2faced4) to head (ce4e263).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #5395   +/-   ##
=====================================
  Coverage   62.1%   62.1%           
=====================================
  Files        186     186           
  Lines      11527   11537   +10     
=====================================
+ Hits        7166    7176   +10     
  Misses      4149    4149           
  Partials     212     212           

Files	Coverage Δ
bridges/prometheus/producer.go	90.6% <100.0%> (+0.3%)	⬆️

[gouthamve]

LGTM