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Metric Factory

The MetricFactory creates an entrypoint to the Metric DSL from an instance of MetricRegistry.

The following examples assume that you have the following imports in scope and that you have constructed a MetricRegistry.

import cats.effect._
import prometheus4cats._

val metricRegistry: MetricRegistry[IO] = MetricRegistry.noop[IO]

Constructing a No-op MetricFactory​

For testing purposes it is possible to construct a MetricFactory whose metrics perform no operations.

MetricFactory.noop[IO]

Constructing from a MetricRegistry​

MetricFactory.builder provides a fluent API that allows you to create an instance that adds an optional prefix and/or common label set to all metrics.

MetricFactory
  .builder
  .build(metricRegistry)

Literal prefixes are checked at compile time to ensure they conform to the OpenMetrics format. Alternatively you may provide an instance of Metric.Prefix that has been refined at runtime.

MetricFactory
  .builder
  .withPrefix("app_name")
  .build(metricRegistry)

Common labels are a set of labels that are checked at runtime so that the label names conform to the OpenMetrics format and no more than ten are defined at any one time, which helps to reduce cardinality.

There is no compile time checking of these labels as it is assumed they will come from the runtime environment.

val commonLabels: Either[String, Metric.CommonLabels] =
  Metric.CommonLabels.ofStrings("name" -> "value")

commonLabels.map { labels =>
  MetricFactory
    .builder
    .withCommonLabels(labels)
    .build(metricRegistry)
}

Changing a MetricFactory​

It is possible to obtain a new instance of a MetricFactory from an existing one with a different/no prefix or common labels.

The snippet below showcases all the syntax for modifying MetricFactory:

val newCommonLabels: Either[String, Metric.CommonLabels] =
  Metric.CommonLabels.ofStrings("name2" -> "value2")

newCommonLabels.map { labels =>
  MetricFactory
    .builder
    .build(metricRegistry)
    .dropPrefix
    .withPrefix("different_prefix")
    .dropCommonLabels
    .withCommonLabels(labels)
}

Transforming the Effect Type (mapK)​

You may transform the effect type (F) of a MetricFactory with a Cats natural transformation (~>).

import cats.~>
import cats.data.EitherT

type F[A] = IO[A]
type G[A] = EitherT[IO, Throwable, A]

val fk: F ~> G = EitherT.liftK[F, Throwable]

val factoryF = MetricFactory.builder.build(metricRegistry)

val factoryG = factoryF.mapK(fk)