tallyotel

A Tally/Open Telemetry bridge allowing for code using Open Telemetry instruments to emit metrics with Tally.

A demonstration of emitting metrics from Open Telemetry instruments through Tally to Prometheus can be found at https://github.com/mmcshane/tallyotel-demo.

As Tally contains abstractions for exporting metrics and Open Telemetry also contains abstractions for exporting metrics, the use of this particular bridge library is likely to be a transient period for any given codebase. If your code is instrumented to use Open Telemetry already then you're probably better off migrating to the Open Telemetry native exporter configuration. However, if you're in a situation where the use of Tally is required but you still want to use Open Telemetry metric instruments, then this library is what you need to bridge the two.

Instrument Support and Mappings

As Tally does not support asynchronous instrument types (i.e. the Observer types), this library does not offer them. Here we describe how Open Telemetry instrument types are mapped to Tally instruments.

OTEL Type	Tally Type	Notes
Counter	tally.Counter	Only integer counters (number.NumberKindInt64) are supported. Note that OTEL counters are monotonic - use UpDownCounter to both increment and decrement.
Asynchronous Counter		Async instruments not supported.
Asynchronous Gauge		Async instruments not supported.
Histogram	tally.Histogram	Histograms using a unit of unit.Millisecond use the Tally Histogram.RecordDuration Histogram API, otherwise Histogram.RecordValue.
UpDownCounter	tally.Counter	Only integer counters (number.NumberKindInt64) are supported.
Asynchronous UpDownCounter		Async instruments not supported.

Tally Scope Usage

Tally makes heavy use of a graph of scope objects for instrument naming and to bundle "tags" with a set of instruments. Open Telemetry does not have a clear analog for Tally scopes but this bridge does use them internally in a predictable way. Here are the rules for how scopes are used within tallyotel.

1. Every tallyotel.MeterProvider instance has a Scope which is passed to it at construction time. Users can use this scope to make Tally-specific configurations to the metrics supplied by the metric.Meter(s) derived from a given top-level tallyotel.MeterProvider
2. Every metric.Meter created by a tallyotel.MeterProvider uses the provided Meter name to build a set of nested sub-scopes (see tally.Scope.SubScope) of the parent tallyotel.MeterProvider's scope. Sub-scopes are implied through the Meter name via a separator string (by default: "."). The exact behavior here can be modified through a client-provided tallyotel.MeterScoper.
3. Instruments created by a metric.Meter and invoked without any attribute.KeyValues use their parent metric.Meter's scope
4. Instruments invoked with attribute.KeyValues use a scope that is a sub-scope of their parent Meter's scope, tagged with the appropriate key-values (see tally.Scope.Tagged)