-spec attach(HandlerId, EventName, Function, Config) -> ok | {error, already_exists}
          when
              HandlerId :: handler_id(),
              EventName :: event_name(),
              Function :: handler_function(),
              Config :: handler_config().

Attaches the handler to the event.

handler_id must be unique, if another handler with the same ID already exists the {error, already_exists} tuple is returned.

See execute/3 to learn how the handlers are invoked.

Note: due to how anonymous functions are implemented in the Erlang VM, it is best to use function captures (i.e. fun mod:fun/4 in Erlang or &Mod.fun/4 in Elixir) as event handlers to achieve maximum performance. In other words, avoid using literal anonymous functions (fun(...) -> ... end or fn ... -> ... end) or local function captures (fun handle_event/4 or &handle_event/4 ) as event handlers.

All the handlers are executed by the process dispatching event. If the function fails (raises, exits or throws) then the handler is removed and a failure event is emitted.

Handler failure events [telemetry, handler, failure] should only be used for monitoring and diagnostic purposes. Re-attaching a failed handler will likely result in the handler failing again.

-spec attach_many(HandlerId, [EventName], Function, Config) -> ok | {error, already_exists}
               when
                   HandlerId :: handler_id(),
                   EventName :: event_name(),
                   Function :: handler_function(),
                   Config :: handler_config().

Attaches the handler to many events.

The handler will be invoked whenever any of the events in the event_names list is emitted. Note that failure of the handler on any of these invocations will detach it from all the events in event_name (the same applies to manual detaching using detach/1).

Note: due to how anonymous functions are implemented in the Erlang VM, it is best to use function captures (i.e. fun mod:fun/4 in Erlang or &Mod.fun/4 in Elixir) as event handlers to achieve maximum performance. In other words, avoid using literal anonymous functions (fun(...) -> ... end or fn ... -> ... end) or local function captures (fun handle_event/4 or &handle_event/4 ) as event handlers.

All the handlers are executed by the process dispatching event. If the function fails (raises, exits or throws) a handler failure event is emitted and then the handler is removed.

Handler failure events [telemetry, handler, failure] should only be used for monitoring and diagnostic purposes. Re-attaching a failed handler will likely result in the handler failing again.

-spec detach(handler_id()) -> ok | {error, not_found}.

Removes the existing handler.

-spec execute(EventName, Measurements) -> ok
           when EventName :: event_name(), Measurements :: event_measurements() | event_value().

Equivalent to execute(EventName, Measurements, #{}).

-spec execute(EventName, Measurements, Metadata) -> ok
           when
               EventName :: event_name(),
               Measurements :: event_measurements() | event_value(),
               Metadata :: event_metadata().

Emits the event, invoking handlers attached to it.

• the event name
• the map of measurements
• the map of event metadata
• the handler configuration given to attach/4

Best practices and conventions:

While you are able to emit messages of any event_name structure, it is recommended that you follow the the guidelines laid out in span/3 if you are capturing start/stop events.

-spec list_handlers(event_prefix()) -> [handler()].

Returns all handlers attached to events with given prefix.

-spec span(event_prefix(), event_metadata(), span_function()) -> span_result().

Runs the provided SpanFunction, emitting start and stop/exception events, invoking the handlers attached to each.

The SpanFunction must return a {result, stop_metadata} tuple.

• EventPrefix ++ [start] and EventPrefix ++ [stop]
• EventPrefix ++ [start] and EventPrefix ++ [exception]

However, note that in case the current process crashes due to an exit signal of another process, then none or only part of those events would be emitted. Below is a breakdown of the measurements and metadata associated with each individual event.

When providing StartMetadata and StopMetadata, these values will be sent independently to start and stop events. If an exception occurs, exception metadata will be merged onto the StartMetadata. In general, it is highly recommended that StopMetadata should include the values from StartMetadata so that handlers, such as those used for metrics, can rely entirely on the stop event. Failure to include all of StartMetadata in StopMetadata can add significant complexity to event handlers.

A default span context is added to event metadata under the telemetry_span_context key if none is provided by the user in the StartMetadata. This context is useful for tracing libraries to identify unique executions of span events within a process to match start, stop, and exception events. Metadata keys, which should be available to both start and stop events need to supplied separately for StartMetadata and StopMetadata.

For telemetry events denoting the start of a larger event, the following data is provided:

• Event:

    EventPrefix ++ [start]

• Measurements:

    #{
      % The current system time in native units from
      % calling: erlang:system_time()
      system_time => integer(),
      monotonic_time => integer(),
    }

• Metadata:

    #{
      telemetry_span_context => term(),
      % User defined metadata as provided in StartMetadata
      ...
    }

• Event:

    EventPrefix ++ [stop]

• Measurements:

    #{
      % The current monotonic time minus the start monotonic time in native units
      % by calling: erlang:monotonic_time() - start_monotonic_time
      duration => integer(),
      monotonic_time => integer()
    }

• Metadata:

    #{
      % An optional error field if the stop event is the result of an error
      % but not necessarily an exception.
      error => term(),
      telemetry_span_context => term(),
      % User defined metadata as provided in StopMetadata
      ...
    }

• Event:

    EventPrefix ++ [exception]

• Measurements:

    #{
      % The current monotonic time minus the start monotonic time in native units
      % derived by calling: erlang:monotonic_time() - start_monotonic_time
      duration => integer(),
      monotonic_time => integer()
    }

• Metadata:

    #{
      kind => throw | error | exit,
      reason => term(),
      stacktrace => list(),
      telemetry_span_context => term(),
      % User defined metadata as provided in StartMetadata
       ...
    }