Add metrics related to watch stream

[hackerwins]

Description:

Currently, Server collects metrics using Prometheus, aggregating changes, operations, and snapshot storage during synchronization(PushPull API). To enhance real-time collaboration features, we propose the addition of the following metrics:

A. Count of Connections

• Purpose: To understand the real-time usage of the service.
• Benefit: This would allow for accurate measurement of real-time collaboration feature usage and better prediction of system load.

B. Event Exchange Size

• Purpose: To monitor the quantity and size of events exchanged through the stream.
• Benefit: This would help optimize network bandwidth usage and improve performance in large collaboration sessions.

Also, if watch stream-related metrics are aggregated by project, it is useful to show graphs on the dashboard later.

Related to yorkie-team/dashboard#162

Why:

With these enhancements, we can better monitor the usage and performance of real-time collaboration features, leading to improved service quality and user experience.

[hackerwins]

@emplam27 Would you like to try to tackle this issue?