[Metricbeat] Add Couchbase Dashboard

[sorantis]

Partially addresses #10594.

This Couchbase dashboard visualizes the following metrics:

  - name: couchbase.cluster.hdd.free.bytes
    description: Free hard drive space in the cluster (bytes).
  - name: couchbase.cluster.hdd.used.by_data.bytes
    description: Hard drive space used by the data in the cluster (bytes).
  - name: couchbase.cluster.hdd.used.value.bytes
    description: Hard drive space used by the cluster (bytes).
  - name: couchbase.cluster.hdd.total.bytes
    description: Total hard drive space available to the cluster (bytes).

  - name: couchbase.cluster.ram.total.bytes
    description: Total RAM available to cluster (bytes).
  - name: couchbase.cluster.ram.used.value.bytes
    description: RAM used by the cluster (bytes).

  - name: couchbase.bucket.memory.used.bytes
    description: Amount of memory used by the bucket (bytes).

  - name: couchbase.bucket.disk.used.bytes
    description: Amount of disk used (bytes).

  - name: couchbase.bucket.ops_per_sec
    description: Number of times a document was read from a database.

  - name: couchdb.server.couchdb.database_writes
    description: Number of operations per second.

  - name: couchbase.bucket.item_count
    description: Number of items associated with the bucket.

  - name: couchbase.bucket.disk.fetches
    description: Number of disk fetches.

  - name: couchbase.node.ops
    description: Number of operations performed on Couchbase.

  - name: couchbase.node.cpu_utilization_rate.pct
    description: The CPU utilization rate (%).

  - name: couchbase.node.couch.docs.disk_size.bytes
    description: Amount of disk space used by Couch docs (bytes).

  - name: couchbase.node.vb_replica_curr_items
    description: Number of items/documents that are replicas.

Dashboard snapshot:

[kaiyan-sheng]

@sorantis We got an error here that CI caught:

2019/08/19 13:16:52 Found Elastic Beats dir at /home/travis/gopath/src/github.com/elastic/beats
>> check: Checking source code for common problems
>> Dashboard format - module/couchbase/_meta/kibana/7/dashboard/Metricbeat-couchbase-overview.json:
   empty description on dashboard '[Metricbeat Couchbase] Cluster Overview'
Error: there are format errors in dashboards

[sorantis]

Fixed