ForwardToPrometheus, create default gauge only if needed

[Christian-Schulz]

The old help text was still published after changing the PublisherOptions in ForwardToPrometheus().

sample:

 services.AddHealthChecks()
   .AddCheck<OwnOracleDBHealthCheck>(OwnOracleDBHealthCheck.HealthCheckName)
   .ForwardToPrometheus(new PrometheusHealthCheckPublisherOptions()
   {
      //override default identifier 'aspnetcore_healthcheck_status'              
      Gauge = Metrics.CreateGauge("own_exporter_healthcheck_status", "Forwarded health check status (0 == Unhealthy, 0.5 == Degraded, 1 == Healthy)", "name")
   });

old output:

# HELP aspnetcore_healthcheck_status ASP.NET Core health check status (0 == Unhealthy, 0.5 == Degraded, 1 == Healthy)
# TYPE aspnetcore_healthcheck_status gauge
# HELP own_exporter_healthcheck_status Forwarded health check status (0 == Unhealthy, 0.5 == Degraded, 1 == Healthy)
# TYPE own_exporter_healthcheck_status gauge
own_exporter_healthcheck_status{name="OracleDBCheck"} 1

new output:

# HELP own_exporter_healthcheck_status Forwarded health check status (0 == Unhealthy, 0.5 == Degraded, 1 == Healthy)
# TYPE own_exporter_healthcheck_status gauge
own_exporter_healthcheck_status{name="OracleDBCheck"} 1

[sandersaares]

Good catch. Thanks for the contribution!