feat(pkg): add go-import page for metrics/exporters/gcp

[aryanmehrotra]

What

Adds the missing vanity-import route for gofr.dev/pkg/gofr/metrics/exporters/gcp.

Why

That path is its own Go module in the gofr repo, but it cannot be fetched from outside the repository — the vanity server returns 404 with no go-import meta tag, so resolution fails before the module's tags are ever consulted:

$ curl -s -o /dev/null -w '%{http_code}\n' 'https://gofr.dev/pkg/gofr/metrics/exporters/gcp?go-get=1'
404

$ curl https://proxy.golang.org/gofr.dev/pkg/gofr/metrics/exporters/gcp/@v/list
not found: unrecognized import path "gofr.dev/pkg/gofr/metrics/exporters/gcp":
reading https://gofr.dev/pkg/gofr/metrics/exporters/gcp?go-get=1: 404 Not Found

Every route under src/app/pkg is hand-written, and the tree only ever covered pkg/gofr/datasource/*. The metrics tree does not exist at all, so /pkg/gofr/metrics, /pkg/gofr/metrics/exporters and the module path itself all 404.

Approach

Identical to #222 (cloudsql, influxdb, kv-store/dynamodb) and #205 (azure):

• layout.jsx — carries the go-import / go-source metadata. Byte-identical to the existing datasource layouts (verified with diff against cloudsql/layout.jsx).
• page.jsx — a PkgRedirect page, so the route actually renders and returns 200. A layout.jsx on its own is not a route in the App Router and would still 404.

docsPath points at /docs/references/configs, which is where METRICS_EXPORTER=gcp is documented. The custom-metrics guide does not mention the exporter.

Verification

Structural clone of a pattern already deployed and working; I did not run a local next build (the prebuild step makes live GitHub calls). Post-merge check:

curl -s -o /dev/null -w '%{http_code}\n' 'https://gofr.dev/pkg/gofr/metrics/exporters/gcp?go-get=1'   # expect 200

Note — this is one half of the fix

Ref gofr-dev/gofr#4112. The module has also never been tagged: the batch of 26 submodule tags was cut at 285d3d2ec (the merge of gofr#3928, the PR that fixed this exporter's go.mod) and covered pkg/gofr/datasource/* only.

This PR must land and deploy first — a tag is useless while the meta lookup 404s. The tag pkg/gofr/metrics/exporters/gcp/v0.1.0 then needs pushing on the gofr repo separately.