Add metric for number of open Workflow Executions

[lorensr]

Is your feature request related to a problem? Please describe.

User request

[ksapchuk]

We would greatly benefit from this feature. We often have bursts of workflows and want to know how far behind we are in processing them. The task schedule latency provides us a rough indication that we are behind but with the metric having a 10 second maximum we have no idea how behind we are.

We could query the number of open workflows periodically to emit our own metric but having one available in the client or server metrics would remove the need of manually emitting one.

[yiminc]

Current recommendation is to do count query on visibility. We will have to think about a long term solution.

[yinsidij]

@yiminc any updates? The number of open workflow (could be emitted as a server metric) would be very helpful and straightforward in the dashboard