Allow specifying multiple mwmbr thresholds

This allows specifying multiple mwmbr thresholds per service.

This is illustrated by converting the deployment thresholds to use this.

Thresholds on a service can be specified using the otherThresholds key. Adding a new threshold will start recording new health metrics under the name gitlab_<threshold name>_health.

For example: specifying the a new threshold as follows:

  otherThresholds: {
    deployment: {
      apdexScore: 0.998,
      errorRatio: 0.9999,
    },
  },

Wil result in the following recordings:

1. The specified thresholds: Scalars with the specified thresholds

   • slo:max:deployment:gitlab_service_errors:ratio
   • slo:min:deployment:gitlab_service_apdex:ratio

2. The health status: Vectors returning 1 if the SLI (error or apdex) meets the threshold, or 0 if it doesn't.

   • gitlab_deployment_health:service:errors
   • gitlab_deployment_health:service:apdex

3. Aggregated health status: Aggregates both SLIs into a health status for the service, or an entire stage.

   • gitlab_deployment_health:service
   • gitlab_deployment_health:stage

This is the groundwork for gitlab-com/gl-infra/mstaff#17 (comment 495326325) after the discussion in !3201 (comment 501271861).