Include mount options in disk metrics

[rul]

This patch just adds the mount options to the disk metrics. This is useful for alerting, for example, when a partition gets remounted as read only due an error.

Required for all PRs:

• [*] Signed CLA.
• [*] Associated README.md updated.
• [*] Has appropriate unit tests.

[danielnelson]

I'm not a big fan of how the options end up in one tag, maybe we should try to extract certain options as tags. What do you think @desa?

[desa]

@danielnelson yeah I'm not a huge fan of that either. I'd just make them separate tags.

[rul]

I wasn't convinced either, but the problem of having the options as separate tags is that not all of them are of the form name=value. Some of them are just a single string, as rw. Do you have any suggestion on what to do in those cases?

[danielnelson]

That does make it trickier to name the tags, and it looks like many options are just a single string. I wonder if instead of trying to support all opts, we should create tags for a smaller set of understood options, which would allow us to customize how it is stored. If we do this we should probably start small, would it be enough in for your use case if you had mode=rw and mode=ro?

[rul]

Oh, yeah, for my use case that's enough. I'll try to update the PR with these changes.

[rul]

@danielnelson I've updated this PR with your suggestion