[Github] Add LLVM Premerge Checks to the watchlist (#120230)

LLVM Premerge Checks is running on the new GCP cluster. Tracking its
metrics will allow us to determine the stability of the presubmit and
make sure the new infra is working as intended.

---------

Signed-off-by: Nathan Gauër <[email protected]>

.ci/metrics/metrics.py

@@ -12,7 +12,7 @@
     "https://influx-prod-13-prod-us-east-0.grafana.net/api/v1/push/influx/write"
 )
 GITHUB_PROJECT = "llvm/llvm-project"
-WORKFLOWS_TO_TRACK = ["Check code formatting"]
+WORKFLOWS_TO_TRACK = ["Check code formatting", "LLVM Premerge Checks"]
 SCRAPE_INTERVAL_SECONDS = 5 * 60