fix(prometheus): quote external_labels values in YAML template (#447)

## Summary
- Add double quotes around `$SERVICE_OWNER`, `$CLUSTER_NAME`,
`$CLUSTER_PEER` and `$PROM_REMOTE_WRITE_TOKEN` in
`prometheus.yml.example`
- Values substituted by `sed` in `run.sh` can contain YAML-special
characters (`#`, `:`, `[`, etc.) that break config parsing
- `$ALERT_DISCORD_IDS` was already quoted — this aligns the rest

## Test plan
- [ ] `docker compose up -d` with `CLUSTER_NAME` containing `#` or `:`
characters
- [ ] Verify prometheus starts and external_labels appear correctly

Author: apham0001

prometheus/prometheus.yml.example

@@ -2,14 +2,14 @@ global:
   scrape_interval: 30s # Set the scrape interval to every 30 seconds.
   evaluation_interval: 30s # Evaluate rules every 30 seconds.
   external_labels:
-    service_owner: $SERVICE_OWNER # replace this with your Operator name you want to be identified by, it helps us route alerts and metrics to your notification channels easily
-    cluster_name: $CLUSTER_NAME
-    cluster_peer: $CLUSTER_PEER
+    service_owner: "$SERVICE_OWNER" # replace this with your Operator name you want to be identified by, it helps us route alerts and metrics to your notification channels easily
+    cluster_name: "$CLUSTER_NAME"
+    cluster_peer: "$CLUSTER_PEER"
 
 remote_write:
   - url: https://vm.monitoring.gcp.obol.tech/write
     authorization:
-      credentials: $PROM_REMOTE_WRITE_TOKEN
+      credentials: "$PROM_REMOTE_WRITE_TOKEN"
     write_relabel_configs:
       - source_labels: [job]
         regex: "charon|mev-boost"