log: add tokio-console support

[hawkw]

This branch adds initial support for setting up the Tokio console to
track tasks in a Kubert controller. In order to do so, I added a new
LogArgs struct in the log module to represent a logging
configuration that can be parsed using clap, including a flag for
enabling tokio-console support. When this flag is present, and the
requisite kubert feature and RUSTFLAGS="--cfg tokio_unstable" are
enabled, the Tokio console server is enabled.

Additionally, the new LogArgs struct handles parsing the LogFormat
and tracing_subscriber::EnvFilter arguments. The LogArgs type uses a
custom clap::FromArgs implementation so that the default value for the
filter can include the name of the binary itself.

Finally, I added support for spawning named tasks using the unstable
tokio::task::Builder API, when the tokio_unstable cfg is enabled.
These task names will show up in the console when the cfg is enabled.

[olix0r]

Unfortunately, using this eliminates the ability to set defaults or configure logging via the environment. I avoided providing a helper so that applications can provide their own variables, defaults, etc.

[hawkw]

I thought that a good middle ground was to set a default that enabled the command name at the info level, which is why there's a manual FromArgs impl. We could also similarly set the env var based on the command name (but uppercased).

If we'd prefer not to do that, we could also pull just the filter out of the struct and have the struct just parse the log format and whether the console should be enabled.

[olix0r]

I think the same issues exist with the log format (with regard to env variables, etc).

Is there any coupling between the tokio-console setting and the other log configuration?

[hawkw]

I think the same issues exist with the log format (with regard to env variables, etc).

Is there any coupling between the tokio-console setting and the other log configuration?

No, it's just also a required argument to the function for actually constructing the tracing subscriber, and I thought it was nice to generate a CLI argument for it as well. We could just as well add a LogBuilder struct or something instead of LogArgs, so that the user can define the CLI args themselves.

[hawkw]

...I just had a crazy thought for how we could do the env vars actually

Edit: oh, nevermind, that's not going to work:

error: `&'static str` is forbidden as the type of a const generic parameter
  --> kubert/src/log.rs:13:34
   |
13 | pub struct LogArgs<const PREFIX: &'static str = "KUBERT"> {
   |                                  ^^^^^^^^^^^^
   |
   = note: the only supported types are integers, `bool` and `char`

sad!

[olix0r]

I think we'll want to add our own macros/main at some point in the future, but for now i'd be inclined to add this new flag as a one-off.

Is the tokio console server configurable at all? especially what port it runs on?