Add dashboard for pubsub metricset in googlecloud module (#21326)

* Add dashboard for pubsub metricset in googlecloud module

Author: kaiyan-sheng

CHANGELOG.next.asciidoc

@@ -718,6 +718,7 @@ https://github.com/elastic/beats/compare/v7.0.0-alpha2...master[Check the HEAD d
 - Sanitize `event.host`. {pull}21022[21022]
 - Add overview and platform health dashboards to Cloud Foundry module. {pull}21124[21124]
 - Release lambda metricset in aws module as GA. {issue}21251[21251] {pull}21255[21255]
+- Add dashboard for pubsub metricset in googlecloud module. {pull}21326[21326] {issue}17137[17137]
 
 *Packetbeat*