Metrics Semantic Conventions

Status: Experimental

The following semantic conventions surrounding metrics are defined:

• HTTP: For HTTP client and server metrics.
• System: For standard system metrics.
• Process: For standard process metrics.
• Runtime Environment: For runtime environment metrics.
• FaaS: For Function as a Service metrics.

Apart from semantic conventions for metrics and traces, OpenTelemetry also defines the concept of overarching Resources with their own Resource Semantic Conventions.

General Guidelines

Metric names and attributes exist within a single universe and a single hierarchy. Metric names and attributes MUST be considered within the universe of all existing metric names. When defining new metric names and attributes, consider the prior art of existing standard metrics and metrics from frameworks/libraries.

Associated metrics SHOULD be nested together in a hierarchy based on their usage. Define a top-level hierarchy for common metric categories: for OS metrics, like CPU and network; for app runtimes, like GC internals. Libraries and frameworks should nest their metrics into a hierarchy as well. This aids in discovery and adhoc comparison. This allows a user to find similar metrics given a certain metric.

The hierarchical structure of metrics defines the namespacing. Supporting OpenTelemetry artifacts define the metric structures and hierarchies for some categories of metrics, and these can assist decisions when creating future metrics.

Common attributes SHOULD be consistently named. This aids in discoverability and disambiguates similar attributes to metric names.

“As a rule of thumb, aggregations over all the dimensions of a given metric SHOULD be meaningful," as Prometheus recommends.

Semantic ambiguity SHOULD be avoided. Use prefixed metric names in cases where similar metrics have significantly different implementations across the breadth of all existing metrics. For example, every garbage collected runtime has slightly different strategies and measures. Using a single set of metric names for GC, not divided by the runtime, could create dissimilar comparisons and confusion for end users. (For example, prefer runtime.java.gc* over runtime.gc.*.) Measures of many operating system metrics are similarly ambiguous.

Units

Conventional metrics or metrics that have their units included in OpenTelemetry metadata (e.g. metric.WithUnit in Go) SHOULD NOT include the units in the metric name. Units may be included when it provides additional meaning to the metric name. Metrics MUST, above all, be understandable and usable.

When building components that interoperate between OpenTelemetry and a system using the OpenMetrics exposition format, use the OpenMetrics Guidelines.

Pluralization

Metric names SHOULD NOT be pluralized, unless the value being recorded represents discrete instances of a countable quantity. Generally, the name SHOULD be pluralized only if the unit of the metric in question is a non-unit (like {faults} or {operations}).

Examples:

• system.filesystem.utilization, http.server.duration, and system.cpu.time should not be pluralized, even if many data points are recorded.
• system.paging.faults, system.disk.operations, and system.network.packets should be pluralized, even if only a single data point is recorded.

General Metric Semantic Conventions

The following semantic conventions aim to keep naming consistent. They provide guidelines for most of the cases in this specification and should be followed for other instruments not explicitly defined in this document.

Instrument Naming

• limit - an instrument that measures the constant, known total amount of something should be called entity.limit. For example, system.memory.limit for the total amount of memory on a system.

• usage - an instrument that measures an amount used out of a known total (limit) amount should be called entity.usage. For example, system.memory.usage with attribute state = used | cached | free | ... for the amount of memory in a each state. Where appropriate, the sum of usage over all attribute values SHOULD be equal to the limit.

  A measure of the amount of an unlimited resource consumed is differentiated from usage.

• utilization - an instrument that measures the fraction of usage out of its limit should be called entity.utilization. For example, system.memory.utilization for the fraction of memory in use. Utilization values are in the range [0, 1].

• time - an instrument that measures passage of time should be called entity.time. For example, system.cpu.time with attribute state = idle | user | system | .... time measurements are not necessarily wall time and can be less than or greater than the real wall time between measurements.

  time instruments are a special case of usage metrics, where the limit can usually be calculated as the sum of time over all attribute values. utilization for time instruments can be derived automatically using metric event timestamps. For example, system.cpu.utilization is defined as the difference in system.cpu.time measurements divided by the elapsed time.

• io - an instrument that measures bidirectional data flow should be called entity.io and have attributes for direction. For example, system.network.io.

• Other instruments that do not fit the above descriptions may be named more freely. For example, system.paging.faults and system.network.packets. Units do not need to be specified in the names since they are included during instrument creation, but can be added if there is ambiguity.

Instrument Units

Units should follow the UCUM (need more clarification in #705).

• Instruments for utilization metrics (that measure the fraction out of a total) are dimensionless and SHOULD use the default unit 1 (the unity).
• Instruments that measure an integer count of something SHOULD use the default unit 1 (the unity) and annotations with curly braces to give additional meaning. For example {packets}, {errors}, {faults}, etc.