Signed-off-by: Eliza Weisman <eliza@buoyant.io>

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

This branch adds a new `Targets` filter to `tracing_subscriber`. The
`Targets` filter is very similar to `EnvFilter`, but it _only_ consists
of filtering directives consisting of a target and level. Because it
doesn't support filtering on field names, span contexts, or field
values, the implementation is *much* simpler, and it doesn't require the
`env_filter` feature flag. Also, `Targets` can easily implement the
`Filter` trait for per-layer filtering, while adding a `Filter`
implementation for `EnvFilter` will require additional effort.

Because the `Targets` filter doesn't allow specifiyng span or
field-value filters, the syntax for parsing one from a string is
significantly simpler than `EnvFilter`'s. Therefore, it can have a very
simple handwritten parser implementation that doesn't require the
`regex` crate. This should be useful for users who are concerned about
the number of dependencies required by `EnvFilter`.

The new implementation is quite small, as it mostly uses the same code
as the static filter subset of `EnvFilter`. This code was factored out
into a shared module for use in both `EnvFilter` and `Targets`. The code
required for _dynamic_ filtering with `EnvFilter` (i.e. on fields and
spans) is still in the `filter::env` module and is only enabled by the
`env-filter` feature flag.

I'm open to renaming the new type; I thought `filter::Targets` seemed
good, but would also be willing to go with `TargetFilter` or something.

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

…o eliza/target-filter

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

Signed-off-by: Eliza Weisman <eliza@buoyant.io>