## Motivation
Currently we can't run
```bash
cargo test -p tracing-subscriber --no-default-features
```
successfully, because there are a bunch of tests for feature flagged
APIs that are always enabled.
## Solution
This commit adds feature flags to the modules and/or individual test
cases that test feature-flagged APIs.
There are still a few doctests that use feature-flagged APIs, and will
fail with `--no-default-features`. These are primarily the examples for
various `Layer::context` methods that rely on `LookupSpan`, and use the
`Registry` type, as it's the only subscriber that *implements*
`LookupSpan`. We could consider changing these examples, either by
removing the actual use of the layers in them, or by changing them to
use a mocked-out version of the registry. However, I think it's nicer to
show how the API would be used in real life. Perhaps we should just run
```bash
cargo test -p tracing-subscriber --no-default-features--tests --lib
```
to ignore doctests when testing without default features.
Signed-off-by: Eliza Weisman <eliza@buoyant.io>
The `FmtCollector` type is missing a `Collect::max_level_hint
method that forwards the inner stack's max level hint.
This fixes that, and adds tests.
Signed-off-by: Eliza Weisman <eliza@buoyant.io>
