[docs] How to write to multiple M3DB clusters

docs/coordinator/api/remote.md

@@ -30,6 +30,10 @@ docs/common/headers_optional_read_write.md
 
 Binary [snappy compressed](http://google.github.io/snappy/) Prometheus [WriteRequest protobuf message](https://github.com/prometheus/prometheus/blob/10444e8b1dc69ffcddab93f09ba8dfa6a4a2fddb/prompb/remote.proto#L22-L24).
 
+### Available Tuning Params 
+
+Refer [here](https://prometheus.io/docs/practices/remote_write/) for an up to date list of remote tuning parameters. 
+
 ### Sample Call
 
 There isn't a straightforward way to Snappy compress and marshal a Prometheus WriteRequest protobuf message using just shell, so this example uses a specific command line utility instead. 

docs/operational_guide/multiple_m3db_clusters.md

@@ -0,0 +1,97 @@
+## Write to multiple M3DB clusters via M3Coordinator
+
+### Overview:
+
+Default M3 architecture has the M3Coordinator writing to and aggregating meterics from a single M3DB cluster. To map a single coordinator to more than one M3DB cluster, follow the below instructions. 
+
+Use case(s):
+- Sending metrics to different namespaces for different retention periods, etc.
+
+### Instructions: 
+
+**Note:** Adding multiple M3DB clusters to m3coordinator using the m3aggregator requires clusterManagement.
+
+1. Add clusterManagement to config file:
+
+Example config file with clusterManagement (see end of the config):
+
+```bash
+clusters:
+  # Should match the namespace(s) set up in the DB nodes
+  - namespaces:
+       - namespace: 21d
+         retention: 504h
+         type: unaggregated
+     client:
+       config:
+         service:
+           env: default_env
+           zone: embedded
+           service: m3db
+           cacheDir: /data/m3kv_default
+           etcdClusters:
+             - zone: embedded
+               endpoints:
+              - <ETCD_IP_1>
+              - <ETCD_IP_2>
+              - <ETCD_IP_3>
+       writeRetry:
+         initialBackoff: 500ms
+         backoffFactor: 3
+         maxRetries: 2
+         jitter: true
+       fetchRetry:
+         initialBackoff: 500ms
+         backoffFactor: 2
+         maxRetries: 3
+         jitter: true
+   - namespaces:
+       - namespace: 90d
+         retention: 2160h
+         type: aggregated
+         resolution: 10m
+       - namespace: 500d
+         retention: 12000h
+         type: aggregated
+         resolution: 1h
+     client:
+       config:
+         service:
+           env: lts_env
+           zone: embedded
+           service: m3db
+           cacheDir: /data/m3kv_lts
+           etcdClusters:
+             - zone: embedded
+               endpoints:
+                 - <ETCD_IP_1>
+                 - <ETCD_IP_2>
+                 - <ETCD_IP_3>
+       writeRetry:
+         initialBackoff: 500ms
+         backoffFactor: 3
+         maxRetries: 2
+         jitter: true
+       fetchRetry:
+         initialBackoff: 500ms
+         backoffFactor: 2
+         maxRetries: 3
+         jitter: true
+tagOptions:
+  idScheme: quoted
+clusterManagement:
+  etcd:
+    env: default_env
+    zone: embedded
+    service: m3db
+    cacheDir: /data/m3kv_default
+    etcdClusters:
+      - zone: embedded
+        endpoints:
+          - <ETCD_IP_1>
+          - <ETCD_IP_2>
+          - <ETCD_IP_3>
+```
+2. Use environment header when calling existing APIs 
+
+**Note:** Any API requests to the coordinator require a `Cluster-Environment-Name` header that corresponds to the cluster the request is targeting.