`Collect()` is called whenever the `/metrics` endpoint is hit to
retrieve metrics.
The queries used in prebuilds metrics collection are quite heavy, and we
want to avoid having them running concurrently / too often to keep db
load down.
Here I'm moving towards a background retrieval of the state required to
set the metrics, which gets invalidated every interval.
Also introduces `coderd_prebuilt_workspaces_metrics_last_updated` which
operators can use to determine when these metrics go stale.
See https://github.com/coder/coder/pull/17789 as well.
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Signed-off-by: Danny Kopping <dannykopping@gmail.com>
