[CI] Add AArch64 Premerge Jobs to Metrics

This enables storing the results in Prometheus/Visualizing them in
Grafana.

Author: boomanaiden154

.ci/metrics/metrics.py

@@ -40,6 +40,7 @@
 GITHUB_JOB_TO_TRACK = {
     "github_llvm_premerge_checks": {
         "Build and Test Linux": "premerge_linux",
+        "Build and Test Linux AArch64": "premerge_linux_aarch64",
         "Build and Test Windows": "premerge_windows",
     },
     "github_libcxx_premerge_checks": {