Add metrics metricset to MongoDB module (#7611)

Add a metricset to cover the metrics field included in the response of `db.serverStatus()`.

Author: a3dho3yn

CHANGELOG.asciidoc

@@ -63,9 +63,9 @@ https://github.com/elastic/beats/compare/v6.4.0...master[Check the HEAD diff]
 
 *Metricbeat*
 - Add metrics about cache size to memcached module {pull}7740[7740]
+- Add `metircs` metricset to MongoDB module. {pull}7611[7611]
 - Add fields for mermory fragmentation, memory allocator stats, copy on write, master-slave status, and active defragmentation to `info` metricset of Redis module. {pull}7695[7695]
 
-
 *Packetbeat*
 
 *Winlogbeat*