Standardised :telemetry event names that every venue package emits. A consumer attaches to these names without coupling to any particular venue's implementation — which is what makes one metrics dashboard work across the whole family.

Telemetry is the metrics channel: high-frequency, individually unimportant, aggregate-only, and lossy by nature. A condition a consumer must act on is not a metric and does not belong here.

Why the category is :link and not :ws

These events were named [:dp_exchange, :ws, …]. That names a transport, and under the facade contract no venue's transport is a consumer's concern — a venue streaming over MQTT or long-polling has no "ws" to report, so it would either emit a lie or emit nothing and look permanently disconnected.

The category is the venue link: the fact that there is a live route to the venue, and whether it is up. What carries that route is package-internal.

Event names

All names are nested lists under [:dp_exchange, <category>, <event>].

Request lifecycle

• [:dp_exchange, :request, :start] — about to fire an outbound request. Measurements: %{system_time: integer}. Metadata: %{provider:, endpoint:, method:}.
• [:dp_exchange, :request, :stop] — request returned (either success or error). Measurements: %{duration: integer (native)}. Metadata: %{provider:, endpoint:, method:, status:, result:}.
• [:dp_exchange, :request, :exception] — request raised an exception. Measurements: %{duration: integer}. Metadata: same as :stop plus %{kind:, reason:, stacktrace:}.

Rate limiting

• [:dp_exchange, :rate_limit, :hit] — caller was rate-limited. Measurements: %{count: 1}. Metadata: %{provider:, retry_after_ms:}.
• [:dp_exchange, :rate_limit, :acquire] — caller acquired tokens. Measurements: %{tokens: pos_integer, wait_ms: non_neg_integer}. Metadata: %{provider:, weight:}.

Link lifecycle

• [:dp_exchange, :link, :up] — a live route to the venue was established. Metadata: %{provider:}.
• [:dp_exchange, :link, :down] — the route was lost. Metadata: %{provider:, reason:}.
• [:dp_exchange, :link, :event] — a payload arrived over the route. Measurements: %{bytes: integer}. Metadata: %{provider:, type:}.
• [:dp_exchange, :link, :reconnect_attempt] — re-establishing after a drop. Metadata: %{provider:, attempt: integer, delay_ms: integer}.

No endpoint metadata: a URL is transport, and a venue with no URL to report would have to invent one.