Add `metrics` request type

We need a request type to match the new `METRICS` pseudo-method in Tide Disco. We can't just `get::` metrics, since they are returned in plain text, but `get` will try to parse a string from JSON or bincode.

We should define a trait `Metrics : tide_disco::Metrics` which adds an `import` method (opposite of `tide_disco::Metrics::export`) which parses a metrics object from a string. Implement this trait for `prometheus::Registry`. Define a `metrcs` request which returns `T`.

Start by locating existing request types in surf-disco and read tide_disco::Metrics::export to understand the corresponding import behavior. Inspect prometheus::Registry parsing support, then define the Metrics trait and metrics request described in the issue. Done means plain-text METRICS responses can be imported into a Registry-backed type and returned through the request.

Written by the indexing model from the issue text.