Downstream url documentation (#4570)

Adds some documentation about downstream_url config in Frontend.

Fixes #3039

Author: srijan55

docs/architecture.md

@@ -214,6 +214,8 @@ Flow of the query in the system when using query-frontend:
 3) Querier picks up the query, and executes it.
 4) Querier sends result back to query-frontend, which then forwards it to the client.
 
+Query frontend can also be used with any Prometheus-API compatible service. In this mode Cortex can be used as an query accelerator with it's caching and splitting features on other prometheus query engines like Thanos Querier or your own Prometheus server.  Query frontend needs to be configured with downstream url address(via the `-frontend.downstream-url` CLI flag), which is the endpoint of the prometheus server intended to be connected with Cortex.
+
 #### Queueing
 
 The query frontend queuing mechanism is used to:

docs/configuration/prometheus-frontend.md

@@ -66,6 +66,6 @@ frontend:
   log_queries_longer_than: 1s
   compress_responses: true
 
-  # The Prometheus URL to which the query-frontend should connect to.
+  # The query endpoint URL of a Prometheus-API compatible service to which the query-frontend should connect to.
   downstream_url: http://prometheus.mydomain.com
 ```

docs/configuration/prometheus-frontend.yml

@@ -22,7 +22,6 @@ query_range:
 
   results_cache:
     cache:
-
       # We're going to use the in-process "FIFO" cache, but you can enable
       # memcached below.
       enable_fifocache: true
@@ -32,7 +31,7 @@ query_range:
 
       # If you want to use a memcached cluster, you can either configure a
       # headless service in Kubernetes and Cortex will discover the individual
-      # instances using a SRV DNS query (host) or list comma separated 
+      # instances using a SRV DNS query (host) or list comma separated
       # memcached addresses.
       # host + service: this is the config you should set when you use the
       # SRV DNS (this is considered stable)
@@ -54,5 +53,5 @@ frontend:
   log_queries_longer_than: 1s
   compress_responses: true
 
-  # The Prometheus URL to which the query-frontend should connect to.
+  # The query endpoint URL of a Prometheus-API compatible service to which the query-frontend should connect to.
   downstream_url: http://prometheus.mydomain.com