rust: add nice command-line argument parsing

[wchargin]

Summary:
This patch outfits //tensorboard/data/server with a nice CLI, courtesy
of clap. The flags are a subset of the main TensorBoard flags, but
with kebab-case rather than snake-case (per everyone-other-than-Google
style, via clap defaults). Since we now accept a --port argument,
which may be 0, we also print the port to which the server is bound.

The clap approach to argument parsing is “define a struct all of whose
fields implement FromStr, then mark it #[derive(Clap)] to generate
an argument parser”. This is great on both the static typing and runtime
performance fronts. To add more arguments, we just add more fields to
struct Opts. See https://docs.rs/clap/3.0.0-beta.2 for details (this
syntax is introduced in clap v3.x, which is still in beta).

Test Plan:
Run bazel run //tensorboard/data/server with -h and --help to see
short and long help text, respectively. Then play around with each
argument in turn.

wchargin-branch: rust-cli

[stephanwlee]

Looks good but I would like to hear your response :)

[stephanwlee]

While I am glad validation is built-in to clap and typing can make sure we do not have, say, negative port number, I am wondering who is in charge of validating the IpAddr. Is it part of IpAddr#FromStr? If so, how about PathBuf?

[wchargin]

Is it part of IpAddr#FromStr? If so, how about PathBuf?

Yep: in all cases, the FromStr implementation is used, so, for example,
<IpAddr as FromStr>, <PathBuf as FromStr>. (On the doc pages,
you can click [src] at right to see the implementations if you want.)

This is also why we impl FromStr for Seconds below.

[wchargin]

Or, maybe this helps: you can tell that the FromStr implementations
are responsible for validation because of the types:

fn from_str(s: &str) -> Result<Self, Self::Err>;

The docs for from_str also say this explicitly, but the signature is
enough to imply it: a method that returns a Result is generally
expected to perform validation of some kind and signal that through the
return value rather than panicking.

[stephanwlee]

Got it, thanks.