Add prometheus service

This allows viewing metrics locally.

Author: jyn514

docker-compose.yml

@@ -9,6 +9,8 @@ services:
             - s3
         ports:
             - "3000:3000"
+        # for metrics
+        expose: ["3000"]
         volumes:
             - "/var/run/docker.sock:/var/run/docker.sock"
             - ".rustwide-docker:/opt/docsrs/rustwide"
@@ -46,6 +48,12 @@ services:
         environment:
             MINIO_ACCESS_KEY: cratesfyi
             MINIO_SECRET_KEY: secret_key
+    prometheus:
+        build:
+            context: ./docker
+            dockerfile: ./Dockerfile-prometheus
+        ports:
+            - "9090:9090"
 volumes:
     postgres-data: {}
     minio-data: {}

docker/Dockerfile-prometheus

@@ -0,0 +1,2 @@
+FROM prom/prometheus
+COPY prometheus.yml /etc/prometheus/

docker/prometheus.yml

@@ -0,0 +1,14 @@
+global:
+  scrape_interval:     15s # By default, scrape targets every 15 seconds.
+
+  # Attach these labels to any time series or alerts when communicating with
+  # external systems (federation, remote storage, Alertmanager).
+  external_labels:
+    monitor: 'docs.rs-monitor'
+
+scrape_configs:
+  # The job name is added as a label `job=<job_name>` to any timeseries scraped from this config.
+  - job_name: 'docs.rs'
+    metrics_path: '/about/metrics'
+    static_configs:
+      - targets: ['web:3000']