[Metricbeat] Support Couchbase SyncGateway

[sorantis]

Describe the enhancement:
Sync Gateway is the synchronization server in a Couchbase for Mobile and Edge deployment. It is designed to provide data synchronization for large-scale interactive web, mobile, and IoT applications
Starting from release 2.8 the new metrics interface in Sync Gateway exposes /_metrics endpoint which exposes stats in Prometheus compatible format.

Add a metricset with the following SyncGatewat metrics:

	Metrics
Resources	CPU Utilization
	Memory Utilization
	Network Transfer - Received
	Network Transfer - Sent
	Overall Heap Usage - HeapAlloc
	Overall Heap Usage - HeapInUse
	Overall Heap Usage - HeapIdle
	Overall Heap Usage - HeapReleased
	Overall Stack Usage
	Garbage Collection time
	Logging - Error
	Logging - Warn
Cache	Channel Cache Utilization - SyncDB Active Revs
	Channel Cache Utilization - SyncDB Removal Revs
	Channel Cache Utilization - SyncDB Tombstone Revs
	Channel Cache Utilization - TransDB Active Revs
	Channel Cache Utilization - TransDB Removal Revs
	Channel Cache Utilization - TransDB Tombstone Revs
	Channel Cache Hits - SyncDB Hits
	Channel Cache Hits - SyncDB Misses
	Channel Cache Hits - TransDB Hits
	Channel Cache Hits - TransDB Misses
	Channel Cache Size - SyncDB
	Channel Cache Size - TransDB
	Channel Cache Hit Ratio
	Revs Cache Performance - SyncDB Hits
	Revs Cache Performance - SyncDB Misses
	Revs Cache Performance - TransDB Hits
	Revs Cache Performance - TransDB Misses
	Revs Cache Hit Ratio (%)
	Channel Cache Count - SyncDB
	Channel Cache Count - TransDB
Database stats	Number of Active Replications - SyncDB
	Number of Active Replications - TransDB
	Rate of new Replications
	Closed Replications
	Rate of document writes - SyncDB
	Rate of document writes - TransDB
	% of docs in conflict
	Rate of document read - SyncDB
	Rate of document read - TransDB
delta_sync	Delta Cache Hit Ratio - SyncDB
	Delta Cache Hit Ratio - TransDB
	Delta Hit/Miss rate - Sent
	Delta Hit/Miss rate - Requested
	Rate of documents sent to SG as a delta
	Rate of pull replications using deltas
	Percentage of pulled documents using delta
	Percentage of pushed documents using delta
Import	Docs Imported - SyncDB
	Docs Imported - TransDB
	Rate of errors as a result of doc import - SyncDB
	Rate of errors as a result of doc import - TransDB
CBL push replication	Average Write Processing Time - SyncDB
	Average Write Processing Time - TransDB
	Rate of documents pushed - SyncDB
	Rate of documents pushed - TransDB
	Average Sync Function Processing Time - SyncDB
	Average Sync Function Processing Time - TransDB
	Average ProposeChange Processing Time - SyncDB
	Average ProposeChange Processing Time - TransDB
	Number of ProposeChange messages - SyncDB
	Number of ProposeChange messages - TransDB
	Rate of attachments pushed - SyncDB
	Rate of attachments pushed - TransDB
	Average size of attachments pushed - SyncDB
	Average size of attachments pushed - TransDB
CBL pull replication	Changes Request Processing Latency - SyncDB
	Changes Request Processing Latency - TransDB
	Server DCP Feed Processing Latency
	Revision Send Message Processing Latency - SyncDB
	Revision Send Message Processing Latency - TransDB
	Rate of attachments pulled - SyncDB
	Rate of attachments pulled - TransDB
	Average size of attachments pulled - SyncDB
	Average size of attachments pulled - TransDB
	Rate of documents pulled in 2.x
	Total Replications by type - SyncDB one shot
	Total Replications by type - SyncDB continuous
	Total Replications by type - TransDB one shot
	Total Replications by type - TransDB continuous
	Active Replications by type - SyncDB one shot
	Active Replications by type - SyncDB continuous
	Active Replications by type - TransDB one shot
	Active Replications by type - TransDB continuous
	Rate of New Initial Replications - SyncDB
	Rate of New Initial Replications - TransDB
	Replications Caught up Count - SyncDB
	Replications Caught up Count - TransDB
Security	Rate of Doc Rejections (by Sync Function) - SyncDB
	Rate of Doc Rejections (by Sync Function) - TransDB
	Rate of Access Failures (by Sync Function) - SyncDB
	Rate of Access Failures (by Sync Function) - TransDB
	Rate of sync Function Auth Failure Count - SyncDB
	Rate of sync Function Auth Failure Count - TransDB
GSI view/query	Rate of channel queries
	Rate of access queries
	Rate of allDocs queries
	Rate of role access queries
	Rate of queries
	Maintenance Queries
	Number of docs transferred
	Number of attachments pushed total
	Attachment bytes transferred
	Number of permanent errors on doc transfers
	Number of documents checked
	Rate of docs transferred

[elasticmachine]

Pinging @elastic/integrations (Team:Integrations)

[sayden]

Ok, here we go elastic/beats#25599

[sorantis]

@sayden nice!

[masci]

Closing as module was merged and backported