Allow custom labels

[dj707chen]

Allow adding custom labels.

Background
In a situation where a service interacts with multiple providers, a few of them do not have a lot of traffic, so when one of these light providers is down, the alert of the service is not triggered due to the small percentage counted in the failing traffic; So we want to create alerts specific to providers, and I’m searching for ways to identify the metrics entries related to a provider.
Two options currently available:

• Use provider specific prefix when create MetricsOps instance, but the metrics of this client will be excluded from a generic Http4S metrics dashboard;
• Add provider specific prefix to classifier when create org.http4s.client.middleware.Metrics.

The second option is better than the first option, but not ideal.

What it does
This PR enables clients to pass custom labels and corresponding values when create MetricsOps and allow alerts targeting those label.

Code refactor
There were a few configuration items (prefix, registry, sampleExemplar, responseDurationSecondsHistogramBuckets), to reduce the number of methods, converted the code to builder pattern, then added withCustomLabelsAndValues method.

Example
Here is an example. If we create a MetricsOps as below and pass it to a client Metrics middleware

  val metricsOpsPaypal = Prometheus.default(registry)
        .withCustomLabelsAndValues(List(("provider", "Paypal"))
        .build
  val paypalClient = BlazeClientBuilder[F]
      .resource
      .map { client => org.http4s.client.middleware.Metrics[F](metricsOpsPaypal)(client) }

Now we can create a Prometheus rule as below, note, label status is set by Metrics middleware, app by filter, and provider is a custom label.

- name: ./team1.rules
   rules:
   - alert: PaypalProviderFailed
      expr: |
         100 * (
             sum by(classifier)(rate(http4s:org_http4s_server_request_count_total{app="banking", provider="Paypal", status="5xx",}[2m]))
             /
             sum by(classifier)(rate(http4s:org_http4s_server_request_count_total{app="banking", provider="Paypal"}[2m]))
         ) > 5
      for: 5m
      labels:
         severity: warning
         team: "team-1"
   - alert: ZelleProviderFailed
      ......

[rossabaker]

Looks good, thanks!