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Metrics API

Table of Content

Metrics API allows to report raw measurements as well as metrics with the known aggregation and labels.

Main class that is used to work with Metrics API is called Meter. It is used to construct Measures to record raw measurements and Metrics to record metrics with predefined aggregation.

Meter

Meter creation

TODO: follow the spec for the Tracer. See work in progress: open-telemetry#39

Create Metric

Meter MUST expose the APIs to create a Metric of every supported type. Depending on the language - builder pattern (C#, Java) or options (Go) SHOULD be used.

Metric creation API requires the following argument.

Required arguments:

  • name of the Metric.
  • type of the Metric. This argument may be implicit from the API method name on Meter was called to create this Metric.

Optional arguments:

  • description of the Metric.
  • unit of the Metric values.
  • list of keys for the labels with dynamic values. Order of the list is important as the same order MUST be used on recording when supplying values for these labels.
  • set of name/value pairs for the labels with the constant values.
  • component name that reports this Metric. See semantic convention for the examples of well-known components.
  • resource this Metric is associated with.

Create Measure

Meter MUST expose the API to create a Measure that will be used for recording raw Measurements.

Depending on the language - builder pattern (C#, Java) or options (Go) SHOULD be used. When multiple Measures with the same arguments were created, implementation may decide to return the same or distinct object. Users of API MUST NOT set any expectations about Measures being unique objects.

Measure creation exposes the following arguments.

Required arguments:

  • name of the Measure.

Optional arguments:

  • description of the Measure.
  • unit of the Measure values.
  • type of the Measure. Can be one of two values - long and double. Default type is double.

Record

Meter provides an API to record Measurements. API built with the idea that Measurementss aggregation will happen asynchronously. Typical library records multiple Measurements at once. Thus API accepts the collection of Measurement so library can batch all the Measurements that needs to be recorded.

Required argument:

  • Set of Measurement to record.

Optional parameters:

  • Explicit DistributedContext to use instead of the current context. Context is used to add dimensions for the resulting Metric calculated out of Measurements.
  • Exemplar of the measurement in a form of SpanContext. This exemplar may be used to provide an example of Spans in specific buckets when histogram aggregation is used.

Measure

Measure is a contract between the library exposing the raw measurement and SDK aggregating these values into the Metric. Measure is constructed from the Meter class, see Create Measure section, by providing set of Measure identifiers.

CreateDoubleMeasurement

Creates a Measurement with the value passes as an argument. It MUST only be called on Measure of a type double. Implementation may return no-op Measurement when there are no subscribers on this Measure. So Measurement cannot be reused and MUST be re-created.

Arguments:

Double value representing the Measurement.

CreateLongMeasurement

Creates a Measurement with the value passes as an argument. It MUST only be called on Measure of a type long. Implementation may return no-op Measurement when there are no subscribers on this Measure. So Measurement cannot be reused and MUST be re-created.

Arguments:

Long value representing the Measurement.

Measurement

Measurement is an empty interface that represents a single value recorded for the Measure. Measurement MUST be treated as immutable short lived object. Instrumentation logic MUST NOT hold on to the object and MUST only record it once.

Metric

Metric is a base class for various types of metrics. Metric is specialized with the type of a time series that Metric holds. Metric is constructed from the Meter class, see Create Metric section, by providing set of Metric identifiers like name and set of label keys.

GetOrCreateTimeSeries

Creates a TimeSeries and returns a TimeSeries if the specified label values is not already associated with this gauge, else returns an existing TimeSeries.

It is recommended to keep a reference to the TimeSeries instead of always calling this method for every operations.

Arguments:

  • List of label values. The order and number of labels MUST match the order and number of label keys used when Metric was created.

GetDefaultTimeSeries

Returns a TimeSeries for a metric with all labels not set (default label value).

Method takes no arguments.

SetCallback

Sets a callback that gets executed every time before exporting this metric. It MUST be used to provide polling of a Metric. Callback implementation MUST set the value of a Metric to the value that will be exported.

RemoveTimeSeries

Removes the TimeSeries from the Metric, if it is present.

Clear

Removes all TimeSeries from the Metric.

Type: Counter

Counter metric aggregates instantaneous values. Cumulative values can go up or stay the same, but can never go down. Cumulative values cannot be negative. TimeSeries for the Counter has two methods - add and set.

  • add adds the given value to the current value. The values cannot be negative.
  • set sets the given value. The value must be larger than the current recorded value. In general should be used in combination with SetCallback where the recorded value is guaranteed to be monotonically increasing.

Type: Gauge

Gauge metric aggregates instantaneous values. Cummulative value can go both up and down. Gauge values can be negative. TimeSeries for the Gauge has two methods - add and set.

  • add adds the given value to the current value. The values can be negative.
  • set sets the given value.