Documented design patterns and code conventions

[pracucci]

What this PR does:
While working on #2204 and #1512 I've realised it's difficult to succeed in the metrics refactoring it it's not a team effort and if we continue to accept PRs breaking the design we're heading to.

For this reason, I would like to propose some basic design patterns and code conventions doc (nothing new to frequent contributors) which we can link to while doing code reviews whenever we see these conventions violated.

What's your take?

Notes:

• I've moved the content of CONTRIBUTING.md under docs/

Which issue(s) this PR fixes:
N/A

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[pstibrany]

LGTM

[gouthamve]

Do we want to adopt Prometheus' style guide as well? https://github.com/prometheus/prometheus/blob/master/CONTRIBUTING.md#contributing

Mainly: Relevant coding style guidelines are the Go Code Review Comments and the _Formatting and style_section of Peter Bourgon's Go: Best Practices for Production Environments.

I have this muscle memory about "comments starting with capitals and ending with period" and other things I've built up since my Prometheus work that I've been suppressing :)

[pstibrany]

I have this muscle memory about "comments starting with capitals and ending with period" and other things I've built up since my Prometheus work that I've been suppressing :)

Thanos has a lint check that verifies this! :)

[pracucci]

Mainly: Relevant coding style guidelines are the Go Code Review Comments and the _Formatting and style_section of Peter Bourgon's Go: Best Practices for Production Environments.

Makes much sense to me. I've added it. @gouthamve What's about now?

[gouthamve]

LGTM

[jtlisi]

LGTM