Add optional namespace restriction to prometheus input plugin

[Benjamintf1]

Allow prometheus to operate as a namespaced pod with namespace level permissions. This addresses #5306

Required for all PRs:

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

[Benjamintf1]

We see that there was a previous issue about this. Seeing their solution, it seems like it could be reasonable/convenient to have it be a toggle and ONLY apply to the namespace of the telegraf config, but we also think the usecases that this supports are wider.

[Benjamintf1]

We seem to be having failed tests unrelated to our changes.. I tried to close/open this pr to run the tests again, this does not seem to re-run tests.

[Benjamintf1]

Done! Although we notice monitor_kubernetes_pods is set to true here, which I do not believe is the default.

[danielnelson]

Thanks, I'll switch that option after we merge this. I don't see your github name in our records as having signed the CLA, could you sign it? If you already signed, sorry something didn't work, its okay to do it again.

[Benjamintf1]

I believe we should be on the pivotal corporate cla. I'm double checking some stuff though as well.

[danielnelson]

I don't see a completed CCLA for Pivotal on my side either, but even so it is also required to do the individual CLA. There is a more info on needing both here: https://www.influxdata.com/legal/ccla/

[Benjamintf1]

Ok, I should have just signed the individual cla.