docs: Update Prometheus metrics config file.

As of Prometheus 2.0, Prometheus metrics must be in the text-based format.

Source: https://prometheus.io/docs/instrumenting/exposition_formats/

wiki/content/deploy/index.md

@@ -1332,11 +1332,11 @@ Dgraph server instances more evenly.
   stage. Increasing this increases memory consumption.
 
 ## Monitoring
-Dgraph exposes metrics via `/debug/vars` endpoint in json format. Dgraph doesn't store the metrics and only exposes the value of the metrics at that instant. You can either poll this endpoint to get the data in your monitoring systems or install **[Prometheus](https://prometheus.io/docs/introduction/install/)**. Replace targets in the below config file with the ip of your Dgraph instances and run prometheus using the command `prometheus -config.file my_config.yaml`.
+Dgraph exposes metrics via the `/debug/vars` endpoint in json format and the `/debug/prometheus_metrics` endpoint in Prometheus's text-based format. Dgraph doesn't store the metrics and only exposes the value of the metrics at that instant. You can either poll this endpoint to get the data in your monitoring systems or install **[Prometheus](https://prometheus.io/docs/introduction/install/)**. Replace targets in the below config file with the ip of your Dgraph instances and run prometheus using the command `prometheus -config.file my_config.yaml`.
 ```sh
 scrape_configs:
   - job_name: "dgraph"
-    metrics_path: "/debug/vars"
+    metrics_path: "/debug/prometheus_metrics"
     scrape_interval: "2s"
     static_configs:
     - targets: