Redis: Adds support for checking length of more types

[Tenzer]

I saw the list length checking feature (#945) was added in the latest version of the check, and thought that I could add some of the code I have from a check I've made to do more or less the same. This adds support for checking other list-like types, namely sets, sorted sets and hashes.

Also, since a key disappears in Redis whenever a list-like item is emptied, it should send a value of 0 to Datadog instead of complaining of the key being missing.

[remh]

Thanks a lot @Tenzer it looks great!

The only point is that if we set up a bad key name there won't be any raised error and it will just send 0 for that key length.

Maybe we should do something like if a key was not found X times in a row we should trigger a warning or something that would be cleared and wouldn't reappear if the key was found at least once ?

What do you think ?

[Tenzer]

In our own use case that wouldn't really work. We are using Redis as a store for Celery and the queue for Celery is a list in Redis. When the queue reaches 0 the key no longer exists in Redis terminology, and that shouldn't be a cause for an error in Datadog.

Making a setting for it on the instance in the configuration could perhaps be an option?

[remh]

That would be a good compromise!
Thanks!

[remh]

Thanks a lot @Tenzer . I'll add the option to warn if the key is missing or not!

[Tenzer]

Thanks for merging. I'm sorry that I didn't get back on this, the last couple of weeks has been crazy busy for me!