chore(deps): update prom/prometheus docker digest to 78ed1f9 (open-telemetry#679)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

prometheus/docker-compose.yml

@@ -5,7 +5,7 @@ services:
     ports:
       - '19090:19090'
   prometheus:
-    image: prom/prometheus@sha256:e2b8aa62b64855956e3ec1e18b4f9387fb6203174a4471936f4662f437f04405
+    image: prom/prometheus@sha256:78ed1f9050eb9eaf766af6e580230b1c4965728650e332cd1ee918c0c4699775
     volumes:
       - ./prometheus.yml:/etc/prometheus/prometheus.yml
     depends_on: