Add more info about filters to docs and rename struct fields

[kgeckhart]

Adjusts the way monitoring.filters are documented and applied.

Similar to #165, I found it hard to use the filters initially so hopefully more detailed docs can help. Especially related to the fact that the monitoring.filters flag should be specified multiple times vs comma delimited like other multi-valued params.

This also adjusts the way the filters are applied to be based on prefix vs contains, https://github.com/prometheus-community/stackdriver_exporter/pull/198/commits/507f6b3119310e14e3a62e66b30100f899e00de0. I think the original contains implementation made it hard to define what exactly belongs in the first section of the extra_filters. Forcing it to be a prefix makes it a lot more concrete.

[BenCoughlan15]

This looks good, is there someone who can review it?

[kgeckhart]

This looks good, is there someone who can review it?

I think only @SuperQ can approve PR's ATM

[BenCoughlan15]

Hey @SuperQ - I'm bumping into this again, It would really save me a tonne of bother if we could get this approved. 🙏

[kgeckhart]

Should be good to go now @SuperQ