feat: add prebuild timing metrics to Prometheus (#19503)

## Description

This PR introduces one counter and two histograms related to workspace
creation and claiming. The goal is to provide clearer observability into
how workspaces are created (regular vs prebuild) and the time cost of
those operations.

### `coderd_workspace_creation_total`

* Metric type: Counter
* Name: `coderd_workspace_creation_total`
* Labels: `organization_name`, `template_name`, `preset_name`

This counter tracks whether a regular workspace (not created from a
prebuild pool) was created using a preset or not.
Currently, we already expose `coderd_prebuilt_workspaces_claimed_total`
for claimed prebuilt workspaces, but we lack a comparable metric for
regular workspace creations. This metric fills that gap, making it
possible to compare regular creations against claims.

Implementation notes:
* Exposed as a `coderd_` metric, consistent with other workspace-related
metrics (e.g. `coderd_api_workspace_latest_build`:
https://github.com/coder/coder/blob/main/coderd/prometheusmetrics/prometheusmetrics.go#L149).
* Every `defaultRefreshRate` (1 minute ), DB query
`GetRegularWorkspaceCreateMetrics` is executed to fetch all regular
workspaces (not created from a prebuild pool).
* The counter is updated with the total from all time (not just since
metric introduction). This differs from the histograms below, which only
accumulate from their introduction forward.

### `coderd_workspace_creation_duration_seconds` &
`coderd_prebuilt_workspace_claim_duration_seconds`

* Metric types: Histogram
* Names:
  * `coderd_workspace_creation_duration_seconds`
* Labels: `organization_name`, `template_name`, `preset_name`, `type`
(`regular`, `prebuild`)
  * `coderd_prebuilt_workspace_claim_duration_seconds`
    * Labels: `organization_name`, `template_name`, `preset_name`

We already have `coderd_provisionerd_workspace_build_timings_seconds`,
which tracks build run times for all workspace builds handled by the
provisioner daemon.
However, in the context of this issue, we are only interested in
creation and claim build times, not all transitions; additionally, this
metric does not include `preset_name`, and adding it there would
significantly increase cardinality. Therefore, separate more focused
metrics are introduced here:
* `coderd_workspace_creation_duration_seconds`: Build time to create a
workspace (either a regular workspace or the build into a prebuild pool,
for prebuild initial provisioning build).
* `coderd_prebuilt_workspace_claim_duration_seconds`: Time to claim a
prebuilt workspace from the pool.

The reason for two separate histograms is that:
* Creation (regular or prebuild): provisioning builds with similar time
magnitude, generally expected to take longer than a claim operation.
* Claim: expected to be a much faster provisioning build.

#### Native histogram usage

Provisioning times vary widely between projects. Using static buckets
risks unbalanced or poorly informative histograms.
To address this, these metrics use [Prometheus native
histograms](https://prometheus.io/docs/specs/native_histograms/):
* First introduced in Prometheus v2.40.0
* Recommended stable usage from v2.45+
* Requires Go client `prometheus/client_golang` v1.15.0+
* Experimental and must be explicitly enabled on the server
(`--enable-feature=native-histograms`)

For compatibility, we also retain a classic bucket definition (aligned
with the existing provisioner metric:
https://github.com/coder/coder/blob/main/provisionerd/provisionerd.go#L182-L189).
* If native histograms are enabled, Prometheus ingests the
high-resolution histogram.
* If not, it falls back to the predefined buckets.

Implementation notes:
* Unlike the counter, these histograms are updated in real-time at
workspace build job completion.
* They reflect data only from the point of introduction forward (no
historical backfill).

## Relates to 

Closes: https://github.com/coder/coder/issues/19528
Native histograms tested in observability stack:
https://github.com/coder/observability/pull/50

coderd/database/queries/workspaces.sql

@@ -923,3 +923,36 @@ SET
 	user_acl = @user_acl
 WHERE
 	id = @id;
+
+-- name: GetRegularWorkspaceCreateMetrics :many
+-- Count regular workspaces: only those whose first successful 'start' build
+-- was not initiated by the prebuild system user.
+WITH first_success_build AS (
+	-- Earliest successful 'start' build per workspace
+	SELECT DISTINCT ON (wb.workspace_id)
+		wb.workspace_id,
+		wb.template_version_preset_id,
+		wb.initiator_id
+	FROM workspace_builds wb
+	JOIN provisioner_jobs pj ON pj.id = wb.job_id
+	WHERE
+		wb.transition = 'start'::workspace_transition
+  		AND pj.job_status = 'succeeded'::provisioner_job_status
+	ORDER BY wb.workspace_id, wb.build_number, wb.id
+)
+SELECT
+	t.name AS template_name,
+	COALESCE(tvp.name, '') AS preset_name,
+	o.name AS organization_name,
+	COUNT(*) AS created_count
+FROM first_success_build fsb
+	JOIN workspaces w ON w.id = fsb.workspace_id
+	JOIN templates t ON t.id = w.template_id
+	LEFT JOIN template_version_presets tvp ON tvp.id = fsb.template_version_preset_id
+	JOIN organizations o ON o.id = w.organization_id
+WHERE
+	NOT t.deleted
+	-- Exclude workspaces whose first successful start was the prebuilds system user
+	AND fsb.initiator_id != 'c42fdf75-3097-471c-8c33-fb52454d81c0'::uuid
+GROUP BY t.name, COALESCE(tvp.name, ''), o.name
+ORDER BY t.name, preset_name, o.name;