add tracing-indicatif as a related crate

[emersonford]

I published the first release of the tracing-indicatif crate that provides a tracing layer to show indicatif progress bars for active spans. @davidbarsky recommended I add this as a related crate to tracing given its potential usefulness to other Rust CLI developers.

Test Plan:
tested with cargo doc --open and the entries are formatted as expected.

[hawkw]

Thanks for adding your crate, this looks very cool!

The docs build failure appears to be unrelated, it looks like a recent nightly must have managed to break our RustDocs. I'll try to figure that out so we can merge your PR.

[davidbarsky]

Subsequent runs of the Netlify job succeeded (e.g., https://app.netlify.com/sites/tracing-rs/deploys/63f54149718158007f46458d), especially after clearing the build cache. I can't tell GitHub that these actions are now passing, nor can I retry them. @emersonford: can you try pushing a dummy commit your branch to convince GitHub to do its thing?

(Don't worry about commit noise, we always squash on merge.)

[davidbarsky]

Quick update:

• Netlify builds are now passing. I'm not sure why they failed in the first place, but I'll chalk it up to a rustdoc regression in nightly.
• CI is now failing on MSRV checks. The issue isn't particularly complex: our current MSRV is 1.49 but opentelemetry-api-0.18 has an MSRV of 1.56 (for posterity, opentelemetry-api's MSRV [on the main branch is 1.57]((but on master, it's been bumped to 1.57: https://github.com/open-telemetry/opentelemetry-rust/blob/main/opentelemetry-api/Cargo.toml#L10)). Unrelated MSRV checks are failing because Cargo from Rust 1.49 does is not aware of the 2021 edition.

@hawkw: I can futz around with the CI, or I can bump tracing's MSRV to 1.57. While Tokio's MSRV is still 1.49, tracing is behind an unstable flag so the impact won't be that bad. Let me know which one you prefer.

[hawkw]

Quick update:

* Netlify builds are now passing. I'm not sure why they failed in the first place, but I'll chalk it up to a rustdoc regression in nightly.

* CI is now failing on MSRV checks. The issue isn't particularly complex: our current MSRV is 1.49 but `opentelemetry-api-0.18` [has an MSRV of 1.56](https://docs.rs/crate/opentelemetry_api/latest/source/Cargo.toml) (for posterity, `opentelemetry-api`'s MSRV [on the main branch is 1.57]((but on master, it's been bumped to 1.57: https://github.com/open-telemetry/opentelemetry-rust/blob/main/opentelemetry-api/Cargo.toml#L10)). Unrelated MSRV checks are failing because Cargo from Rust 1.49 does is not aware of the 2021 edition.

@hawkw: I can futz around with the CI, or I can bump tracing's MSRV to 1.57. While Tokio's MSRV is still 1.49, tracing is behind an unstable flag so the impact won't be that bad. Let me know which one you prefer.

We shouldn't do a global MSRV bump for a dependency of tracing-opentelemetry, since nothing else depends on opentelemetry-api. Instead, we should bump only tracing-opentelemetry 's MSRV. I believe tracing-opentelemetry already has a newer MSRV than tracing-core, tracing, and friends, so it should be pretty straightforward to bump just that crate --- the CI configs are already testing it against a different MSRV, so you can just increase that.