Sharding: document supported relabel action and add store gateway bac…

…kgroud (#2272)

* Sharding: document supported relabel action and add store gateway background

Signed-off-by: Xiang Dai <764524258@qq.com>

* add hashmod

Signed-off-by: Xiang Dai <764524258@qq.com>

docs/sharding.md

@@ -17,11 +17,21 @@ This means that with time you might want to scale out certain components e.g:
 
 * Compactor: Larger number of objects does not matter much, however compactor has to scale (CPU, network, disk, memory) with number of Sources pushing blocks to the object storage.
 
+* Store Gateway: Queries against store gateway which are touching large number of Sources might be expensive, so it has to scale up with number of Sources if we assume those queries.
+    * Orthogonally we did not advertise any labels on Store Gateway's Info. This means that querier was not able to do any pre-filtering, so all store gateways in system are always touched for each query.
+
 # Relabelling
 
 Similar to [promtail](https://github.com/grafana/loki/blob/master/docs/promtail.md#scrape-configs) this config will follow native
 [Prometheus relabel-config](https://prometheus.io/docs/prometheus/latest/configuration/configuration/#relabel_config) syntax.
 
+Now, thanos only support following relabel actions:
+* keep
+* drop
+* hashmod
+    * `externa labels` for all components
+    * `__block_id` for store gateway, see this [example](https://github.com/observatorium/configuration/blob/master/environments/openshift/manifests/observatorium-template.yaml#L1511)
+
 The relabel config defines filtering process done on **every** synchronization with object storage.
 
 We will allow potentially manipulating with several of inputs: