Disable prometheus metrics by default

The HTTP server should only be enabled if a user explicitly sets a
port for it.

README.md

@@ -100,6 +100,17 @@ dbs:
       - path: /path/to/replica
 ```
 
+### Monitoring replication
+
+You can also enable a Prometheus metrics endpoint to monitor replication by
+specifying a bind address with the `addr` field:
+
+```yml
+addr: ":9090"
+```
+
+This will make metrics available at: http://localhost:9090/metrics
+
 
 ### Other configuration options
 
@@ -119,6 +130,7 @@ These replica options are only available for S3 replicas:
 - `sync-interval`—Replication sync frequency.
 
 
+
 ## Usage
 
 ### Replication