admin: add /stats filtering capability

[ambuc]

Description: Added /stats stat name filtering. You can now hit the endpoint /stats?filter=foo and get just counters/gauges/histograms which have "foo" in their names. Compatible with json and used-only modes. Comes from an idea discussed in #4361. This doesn't support regex, just relies on absl::StrContains() under the hood.
Risk Level: Low.
Testing: Added two tests to admin_test.cc. Now tests the full space of filtering by used-only and/or filtering by string.
Docs Changes: I edited admin.rst to reflect this.
Release Notes: N/A

[ggreenway]

While we're adding this, should we just make it a regex so it covers more use cases and we don't need to support multiple options later?

[ambuc]

@ggreenway I'm not opposed to this at all -- but I welcome recommendations on how to a) do this "efficiently", and b) how to encode the usual complexity of a regex in a URL. Is this done elsewhere in Envoy?

[ggreenway]

a) I'm assuming that really simple regex will be fast enough, but that may be incorrect
b) percent/url encoding, although that may be really painful for CLI, although it looks like curl will do this for you: https://unix.stackexchange.com/questions/86729/any-way-to-encode-the-url-in-curl-command

But if the performance is too poor, even for a simple regex like "cluster", we can leave this as-is, and add another query-param for filtertype=regex later, or something.

[ambuc]

@ggreenway You were right, this is pretty easy to implement. Changes performed in 280029d.

[ambuc]

@ggreenway, requesting review on this PR (now that I've kicked CircleCI).

[ramaraochavali]

Thanks. LGTM other than one minor function name change nit .

[ambuc]

@ramaraochavali I made the change in c020e2a, thanks!

[ramaraochavali]

Thanks. LGTM

[ggreenway]

Looks good. Thanks for doing the work to handle regex.