Collect Worker Count Metric that Runs Through the Background Package

[dc7303]

What would you like to be added:

yorkie/server/backend/background/background.go

Line 45 in d016530

func (b *Background) AttachGoroutine(f func(ctx context.Context)) {

I think you can try to collect the count of workers running in the background package.

Why is this needed:
You can know ahead of time when to process many workers. This can prevent problems that may arise from this.

[hackerwins]

Thank you for reporting. It would be helpful to examine these metrics. Feel free to submit a PR.

[kokodak]

I'm interested in this issue, would the following rough approach be correct?

1. create a counter in the AttachGoroutine() method to record when the goroutine starts or ends.
2. collect counter metrics to show a monitoring tool like Prometheus how many goroutine workers are running in the background.

[krapie]

@kokodak Yes. I will assign you on this issue 👍