Generic :telemetry handler for apps whose failure events follow the canonical metadata schema: stage, error_class, error_message.

Scoped to 3-segment failure events ([:<app>, :<domain>, :failure]).

Usage

# In your Application.start/2 or a Telemetry supervisor:
FosferonTelemetry.FailureHandler.attach(:my_app, [:session, :checkout, :support])

This attaches a single handler via :telemetry.attach_many/4 that logs all matching events at :warning level with the canonical metadata fields.

Idempotency

attach/2 is idempotent — :already_exists from :telemetry.attach_many/4 is treated as :ok, so application restarts and supervision re-init are safe.

Self-recovery

The handler rescues its own exceptions to prevent :telemetry from detaching it on crash. A crashing handler would silently drop all subsequent failure events for the app.