Lists metrics using bidirectional cursor pagination: Whippy's built-in metrics first, in a
fixed order, followed by the organization's saved metrics (newest first).

A metric is a single chart (one configured measurement). Reports group metrics together; use
the Reports endpoints to list and run those.

Built-in metrics (source: whippy) are the analytics metrics that make up the built-in
Agents, Conversations, Campaigns, Calls, and Usage dashboards. They have stable reserved IDs
(prefix 00000000-0000-4000-) and null created_at/updated_at. Saved metrics
(source: organization) are sorted by created_at descending, with id as a tiebreaker.

Pagination

• First page: omit cursor (or use direction=forward, the default).
• Next page: use metadata.next_cursor with direction=forward.
• Previous page: use metadata.prev_cursor with direction=backward.
• direction=backward without cursor: returns 422.
• offset: not supported.

Cursors are opaque, base64-encoded tokens that decode to {"inserted_at": "<ISO8601>", "id": "<uuid>"}.

Filtering

• source — optionally filter by whippy (built-in metrics) or organization (metrics
  saved in the organization). Omitting it returns both.
• type — optionally filter by custom (metrics created in the UI) or non_custom
  (metrics backed by a report defined internally by Whippy).

source and type are independent filters and combine. Built-in metrics are all
non_custom, so type=custom excludes them, while type=non_custom returns built-in
metrics alongside saved non_custom metrics.