feat: Add Summary metric type

[tisonkun]

This closes #40.

This follows up #67.

With ...

• https://github.com/prometheus/client_rust/pull/67/files#r1145927745 Resolved. All related code is behind "summary" feature now.
• https://github.com/prometheus/client_rust/pull/67/files#r1145930654 Resolved. crate instant is not maintained any more, using fastant which works on wasm also.
• https://github.com/prometheus/client_rust/pull/67/files#r1145938371 Resolved. TRIVIAL AS IS.
• https://github.com/prometheus/client_rust/pull/67/files#r1145924555 Resolved. Use the same write lock when needed. See 22865b4.
• https://github.com/prometheus/client_rust/pull/67/files#r1145932768 Resolved. TRIVIAL AS IS.
• https://github.com/prometheus/client_rust/pull/67/files#r1145935223 Not accepted. We may improve this point later. Currently it brings extra burden without clearly benefit.

[tisonkun]

cc @mxinden

[cratelyn]

if we add this line...

Suggested change

	#[cfg(feature = "summary")]
	pub mod summary;
	#[cfg(feature = "summary")]
	#[cfg_attr(docsrs, doc(cfg(feature = "macros")))]
	pub mod summary;

we'll also make sure that this shows up in the documentation generated on docs.rs, see: https://doc.rust-lang.org/rustdoc/unstable-features.html#doccfg-recording-what-platforms-or-features-are-required-for-code-to-be-present

n.b. we'd need to also add this above in lib.rs, if it's not already present:

#![cfg_attr(docsrs, feature(doc_cfg))]

[tisonkun]

I use this trick for my crates and I'm glad to include this.

However, it seems a separated topic to this PR so I'd like to listen to maintainer's idea whether we should include it in this PR or add in a follow-up. Both works for me but different maintainers would have different preference.