Add optional support for tracing besides log

[8573]

As an optional feature, add support for getting the verbosity level as
a tracing::Level to pass to Tokio's tracing library, very similar
to the existing support for getting the verbosity level as a
log::Level.

A tracing equivalent of the existing Verbosity::log_level_filter
method easily could be added as well:

/// Get the log level filter, as defined by the `tracing` crate.
pub fn tracing_level_filter(&self) -> tracing::level_filters::LevelFilter {
    self.tracing_level().into()
}

However, that method's body is so simple that I question whether
adding it would be worth your potentially needing to update it if
tracing, for example, moves the LevelFilter type out of that
level_filters module.

[epage]

How mature / stable is tracing for being a 0.1 crate? We just went 1.0 and I don't want to repeatedly bump by taking on a potentially volatile dependency

[8573]

How mature / stable is tracing for being a 0.1 crate?

I'm only a very new user of tracing, so I don't know. On one hand, I would guess that tracing::Level is too basic to be likely to disappear. On the other hand, in hindsight, considering the crates' very different version numbers, I do think it would be more appropriate for me to ask tracing to add the necessary conversion code.

[epage]

On one hand, I would guess that tracing::Level is too basic to be likely to disappear.

My concern is not with it disappearing but that when a tracing 0.2 comes out, to support it, we'd have to release clap-verbosity-flag 2.0. Ditto for each breaking release of tracing, regardless of whether the type looks the same. Rust will consider tracing::Level from 0.1, 0.2, 0.3, etc all to be different, incompatible types.

[epage]

How do tracing users generally feel about still requiring log? If a breaking change is involved in the future, should we allow disabling it?

[8573]

How do tracing users generally feel about still requiring log?

I'm only a very new user of tracing, so I don't know, but I note that the main tracing README suggests that applications use the crate tracing_subscriber, which already, as a default feature, depends on log and consumes log messages, and the main tracing crate also interfaces with log as a non-default feature.

I'm sure some users would consider the cost in compilation time of log to be too much, but would they be using clap-derive?

[epage]

For tracing, is it more appropriate to return Level or LevelFilter?

[epage]

To be clear, I'm not saying "also" but "instead of"

[8573]

They're very easily convertible to each other, and the equivalent of filter_level accepts both, so I don't think it matters much. I chose tracing::Level over tracing::level_filters::LevelFilter because I guessed that, if either's path were to change in the future, it more likely would be the latter's.

[8573]

On one hand, I would guess that tracing::Level is too basic to be likely to disappear.

My concern is not with it disappearing but that when a tracing 0.2 comes out, to support it, we'd have to release clap-verbosity-flag 2.0. Ditto for each breaking release of tracing, regardless of whether the type looks the same.

I hadn't thought of that. That thought convinces me against proposing to add this support in this library, rather than proposing to add a conversion from log's types in tracing.

[Zykino]

FYI tokio/tracing maintain an (unstable) crate called tracing-log for the conversion between log and tracing. Maybe as a user you could use that? I understand that depending on an unstable crate is not ideal here.
https://github.com/tokio-rs/tracing#project-layout

[epage]

#39 at least added an example of integrating with tracing. I don't use it, so no idea how idiomatic it is.

[joshka]

Issue raised for further discussion: #121