feat(subscriber) add filter env var builder parameter

[mujpao]

Add a new builder method to console_subscriber::Builder to configure the filter environment variable to use for tracing events.

closes #206

[hawkw]

Overall, the implementation looks right. I had some naming and documentation suggestions.

It would probably also be good to mention something about this method in the documentation for Builder::init, where it discusses the RUST_LOG environment variable:

console/console-subscriber/src/builder.rs

Lines 205 to 209 in a77c6f4

	/// In addition to the [`ConsoleLayer`], which collects instrumentation data
	/// consumed by the console, the default [`Subscriber`][sub] initialized by this
	/// function also includes a [`tracing_subscriber::fmt`] layer, which logs
	/// tracing spans and events to stdout. Which spans and events are logged will
	/// be determined by the `RUST_LOG` environment variable.

and maybe also here:

console/console-subscriber/src/builder.rs

Line 237 in a77c6f4

/// | `RUST_LOG` | Configures what events are logged events. See [`Targets`] for details. | "error" |

it would be good if that documentation mentioned that the environment variable can be changed using this method.

Thanks for working on this; once the docs and naming changes are addressed, I think this will be good to merge!

[hawkw]

I think maybe we should rename this to filter_env_var rather than filter_env_variable? It feels a little bit weird to me that we abbreviate "environment" to "env" but we don't abbreviate "variable" to "var", especially since the stdlib function is std::env::var rather than std::env::variable.

i know i was the one who suggested this name originally, sorry about that.

[hawkw]

i went ahead and made the remaining changes, this LGTM!