[mongo] add mongodb.rs_status service status for check state all replset members

[mpidlisnyi]

We want to control state of all members of replset. We should make sure that replset is healthy even we have only one DD agent on the one of replset nodes. For example during extension of cluster or shrinking. Our cluster has a lot of shards each has own replset. Therefore checking state of all replset members for each agent is useful.
We made a small patch with additional service check and we want to know your opinion about that. Would be great if you accept our code in official datadog agent repo.
Thank you

[gmmeyer]

Hey @mpidlisnyi!

Thank you very much for your contribution! Unfortunately, this is a pretty big change and adds a new feature that is reasonably complex, so we're not going to be able to merge it before the next release. In the next release, we're moving to a slightly different model for the agent checks, where we're taking them out of this repo, and testing that to ensure it works will take up a good bit of our time.

Thank you again, your contribution is definitely appreciated :)

[irabinovitch]

@mpidlisnyi Thanks for this PR! As Greg mentioned we're currently migrating checks out this repo and into new https://github.com/DataDog/integrations-core and https://github.com/DataDog/integrations-extras. This change would fit into the https://github.com/DataDog/integrations-core repo.

The will enable a new release process that allows us to test, merge, and release integrations individually rather than waiting on a full dd-agent release.

Would it be OK if we close this PR and and as you to open a new one against the integrations-core repository?

Thank you for your patience as we have been working through this transition. If you have any questions, we are happy to discuss here in this PR or on Slack in #integrations. Our next community office hours and code review is tomorrow at 10:30am PT and we'd be happy to review a PR with you there if you'd like to join us.