)

The `provider` label was inconsistent between AI Gateway metrics. Every
metric emitted by the gateway labels `provider` with the provider
instance name, for example `anthropic-eu`, while
`coder_ai_gateway_cost_control_unpriced_token_usage_records_total` used
the provider type, for example `anthropic`. The two could not be
correlated on `provider`.

The metric was also inconsistent with itself: the path where a provider
fails to resolve labelled by instance name, and the path where a model
has no price labelled by type. The type is still worth exposing, since
prices are keyed on `(provider_type, model)` and that is what an
operator needs to add a price.

- Label the metric with `provider` (the instance name, consistent with
the other gateway metrics) and add `provider_type` (the configured type
the price is keyed on).
- Use `unknown` for `provider_type` when the provider does not resolve
to a configured type.
- Log the unresolved-provider case at `warn` instead of `info`. A
missing price is an expected steady state, but a provider that cannot be
resolved is not.
- Update the metrics docs and the `metricsdocgen` fixture.

Closes [AIGOV-574](https://linear.app/codercom/issue/AIGOV-574)

> [!NOTE]
> Initially generated by Claude Opus 5, modified and reviewed by
@ssncferreira

(cherry picked from commit 95328f1)