subscriber: emit deprecation warnings for "filter"

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

tracing-subscriber/src/filter/env/mod.rs

@@ -29,6 +29,13 @@ use tracing_core::{
 /// A `Layer` which filters spans and events based on a set of filter
 /// directives.
 // TODO(eliza): document filter directive syntax?
+#[cfg_attr(
+    feature = "filter",
+    deprecated(
+        since = "0.1.2",
+        note = "the `filter` feature flag was renamed to `env-filter` and will be removed in 0.2",
+    )
+)]
 #[derive(Debug)]
 pub struct EnvFilter {
     // TODO: eventually, this should be exposed by the registry.