PMM-12712 Doc about new shards collector. (#778)

* PMM-12712 Doc about new shards collector.

* PMM-12712 Typo.

Author: JiriCtvrtka

README.md

@@ -128,6 +128,17 @@ Usage example: `db.setProfilingLevel(2)`
 |1| The profiler collects data for operations that take longer than the value of `slowms` or that match a filter.<br> When a filter is set: <ul><li> The `slowms` and `sampleRate` options are not used for profiling.</li><li>The profiler only captures operations that match the filter.</li></ul>
 |2|The profiler collects data for all operations.|
 
+#### Enabling shards metrics gathering
+When shard metrics collection is enabled by `--collector.shards`, the exporter will expose metrics related to sharded Mongo. 
+Example, if shards collector is enabled:
+```
+# HELP mongodb_shards_collection_chunks_count sharded collection chunks.
+# TYPE mongodb_shards_collection_chunks_count counter
+mongodb_shards_collection_chunks_count{collection="system.sessions",database="config",shard="rs1"} 250
+mongodb_shards_collection_chunks_count{collection="system.sessions",database="config",shard="rs2"} 250
+```
+You can see shard name, it's collection, database and count.
+
 #### Cluster role labels
 The exporter sets some topology labels in all metrics.
 The labels are:

REFERENCE.md

@@ -27,5 +27,6 @@
 |--collector.collstats-limit=0|Disable collstats, dbstats, topmetrics and indexstats collector if there are more than \<n\> collections. 0=No limit|
 |--collector.profile-time-ts=30|Set time for scrape slow queries| This interval must be synchronized with the Prometheus scrape interval|
 |--collector.profile|Enable collecting metrics from profile|
+|--collector.shards|Enable collecting metrics related to Mongo shards|
 |--metrics.overridedescendingindex| Enable descending index name override to replace -1 with _DESC ||
 |--version|Show version and exit|