Support Accept: application/json on common HTTP endpoints

[joe-elliott]

What this PR does:

• Adds support for the Accept: application/json header to the following endpoints:

  /distributor/all_user_stats
  /distributor/ha_tracker
  /ingester/ring
  /store-gateway/ring
  /compactor/ring
  /ruler/ring
  /services

• Renders the /services endpoint in the standard Cortex html style

Which issue(s) this PR fixes:

Checklist

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

[joe-elliott]

Parsing Accept correctly is non-trivial:
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Accept

More generally there is some discussion about adding support for content negotiation to the standard library here:
golang/go#19307

However, for our purposes I think a simple string.Contains is sufficient since there is no standard implementation, but I would be willing to research and use an existing implementation of Accept parsing if desired.

[pstibrany]

Our V1 Guarantees page explicitly says

• Additional API endpoints for management of Cortex itself, such as the ring. These APIs are not part of the any compatibility guarantees.

If we are clear that these endpoints may change and break at any point, we can keep them.

Proper handling of accepts (with priorities) would be nice, but since browsers are typically not setting application/json, this should be fine for now.

[gouthamve]

LGTM!

[pracucci]

LGTM, thanks!