Deploy containerized Prometheus for gnmic telemetry

Add prometheus node to the Containerlab topology, scraping the gnmic
exporter (172.16.0.70:9273) at 5s interval with relabeling to clean
device/interface/neighbor_address/vlan/mac_address/afi_safi labels.
Validated against BGP and system metrics confirmed in #44. Existing
external Prometheus instance stays untouched in parallel.

Refs #45

evpn-lab.clab.yml

@@ -302,6 +302,13 @@ topology:
                 - configs/gnmic/gnmic-config.yml:/gnmic-config.yml:ro
             cmd: --config /gnmic-config.yml --log subscribe
 
+        prometheus:
+            kind: linux
+            mgmt-ipv4: 172.16.0.71
+            image: prom/prometheus:v2.55.1
+            binds:
+                - configs/prometheus/prometheus.yml:/etc/prometheus/prometheus.yml:ro
+
     links:
         # =====================================================
         # DATACENTER FABRIC LINKS