Make method env_var_name public

[LucDrenth]

Feature Request

I would like to make tracing-subscriber/src/filter/env/builder.rs::env_var_name a public method.

Crates

tracing-subscriber

Motivation

I would like to use the builder and check if there is an environment variable set. This requests comes from the following Bevy engine issue: bevyengine/bevy#13897

Proposal

Make the method public.

Alternatives

I can use EnvFilter::DEFAULT_ENV since that is expoed but then I can not use the builder its env property. I can avoid using the builder as a workaround but that is not ideal since it would require me to duplicate code.

[alice-i-cecile]

Make tracing-subscriber/src/filter/env/builder.rs::env_var_name a public method. I would like to

Sentence fragment here :)