Modify response to have via and server headers

[aryan9600]

This PR addresses linkerd/linkerd2#2295.
I am not very confident about this PR, hence the draft PR.
Also, a rather peculiar thing, checking the response via the viz dashboard doesn't show the via or server header, but the debug statements do confirm that the response has been modified. It'd be great if I could get some feedback on how to improve this.

[olix0r]

@aryan9600 What do you think about omitting the via header for now? Users may not necessarily want to expose this information on requests so I'd like to be a bit careful about introducing that.

I think adding a server header to Linkerd-terminated requests seems like a good compromise, though; as this information is necessary to effectively debug 5xx responses.

[hawkw]

thanks for working on this! i agree with @olix0r's comments about the via header. also, i left a few suggestions about the server header inline.

[hawkw]

i don't think it's really correct to use the cargo package version as the proxy's version. this is going to be the version of the linkerd-app-core crate, which isn't the actual proxy version: https://github.com/aryan9600/linkerd2-proxy/blob/6438d0d19dafc22d6645d198b566085c7c5d0e73/linkerd/app/core/Cargo.toml#L3

instead, we should probably use the GIT_VERSION env var, which is the current proxy version git tag, set by the build script:

linkerd2-proxy/linkerd/app/core/build.rs

Lines 24 to 27 in 1223447

	set_env(
	"GIT_VERSION",
	Command::new("git").args(&["describe", "--always", "HEAD"]),
	);

Suggested change

	const SERVER_HEADER: &'static str = concat!("linkerd-proxy/", env!("CARGO_PKG_VERSION"));
	const SERVER_HEADER: &'static str = concat!("linkerd-proxy/", env!("GIT_VERSION"));

[hawkw]

the linting checks on CI are failing because the clippy linter doesn't like the explicit 'static lifetime here (constants always have the 'static lifetime). to get CI passing, we should probably remove it:

Suggested change

	const SERVER_HEADER: &'static str = concat!("linkerd-proxy/", env!("CARGO_PKG_VERSION"));
	const SERVER_HEADER: &str = concat!("linkerd-proxy/", env!("CARGO_PKG_VERSION"));

[aryan9600]

oops, forgot to consult clippy before pushing 😅

[aryan9600]

Users may not necessarily want to expose this information on requests so I'd like to be a bit careful about introducing that.

What if we keep it disabled by default and add an opt-in annotation to enable this? @olix0r @hawkw