Generates Telemetry.Metrics.counter/2 definitions for failure events.

Two input modes cover 3-segment and 4–5 segment event names.

Shorthand mode (3-segment apps)

For apps where every failure event follows the [:<app>, :<domain>, :failure] convention:

FosferonTelemetry.FailureMetrics.counters(:my_app, [:session, :checkout, :support])
# => [
#   counter("my_app.session.failure.count", tags: [:stage]),
#   counter("my_app.checkout.failure.count", tags: [:stage]),
#   counter("my_app.support.failure.count", tags: [:stage])
# ]

Explicit tuple mode (4–5 segment apps)

For apps with non-standard event structures, pass a list of {event_name, metric_name} tuples:

FosferonTelemetry.FailureMetrics.counters([
  {[:billing, :invoices, :refund, :error],
   "billing.invoices.refund.error.count"},
  {[:billing, :invoices, :line_items, :create, :error],
   "billing.invoices.line_items.create.error.count"}
])

Tag discipline

v1 rule: counters use only :stage as a tag. Never tag on IDs, slugs, or free-text fields — unbounded cardinality kills Prometheus.