metrics: Add nil check for metricsHandler in AdminMetrics.serveHTTP (#7553)

* metrics: Add nil check for metricsHandler in AdminMetrics.serveHTTP

Prevents panic when the admin metrics endpoint is accessed before
the module is fully provisioned. Returns a proper API error instead
of crashing.

* admin: provision router modules before registering routes

Instead of adding a nil check for metricsHandler, address the root
cause by provisioning admin router modules before calling Routes().
This ensures all handler state is initialized before routes are
registered on the mux.

Merge newAdminHandler and provisionAdminRouters into a single step,
removing the two-phase setup where routes were registered first and
modules provisioned later. The AdminConfig.routers field is no longer
needed since provisioning happens inline.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* fix: go fmt admin.go

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>