Fix exemplars being added to gauge metrics in the prometheus exporter

[trthomps]

Prometheus Gauge metrics don't support exemplars and while addGaugeMetric() doesn't add them, addSumMetric() will if the metric is monotonic. This causes the prometheus client to throw an error:

* error collecting metric Desc{fqName: "http_server_request_body_size_bytes", help: "Measures size of RPC request messages (uncompressed).", constLabels: {}, variableLabels: {net_protocol_name,net_protocol_version,http_method,http_route,http_scheme,net_host_name,net_host_port,otel_scope_name,otel_scope_version}}: cannot inject exemplar into Gauge, Summary or Untyped

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: trthomps / name: Travis Thompson (7f366df, ab6a89e, b9cdef3, 88cd690, b664503, 0e82181, fdc2cb8, 11b88a7, 2d16850, 4caf61e, e841bbb)
• ✅ login: dmathieu / name: Damien Mathieu (3700b79)

[dashpole]

Would be good to have a test

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.6%. Comparing base (30c4a9a) to head (3700b79).
Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #5912   +/-   ##
=====================================
  Coverage   84.6%   84.6%           
=====================================
  Files        272     272           
  Lines      22837   22839    +2     
=====================================
+ Hits       19322   19325    +3     
+ Misses      3171    3170    -1     
  Partials     344     344           

see 2 files with indirect coverage changes

[trthomps]

Would be good to have a test

@dashpole Added a test, let me know if you know of a better way to test this, I did step through the debugger and exemplars are getting added to the DataPoint.

[trthomps]

Thanks so much for working with me on this @dashpole.