Fix typespec for drop option

I forgot to update this in 3fde830. Now we're using the same `keep_fun` type for both `keep` and `drop` to prevent them getting out-of-sync.

lib/telemetry_metrics.ex

@@ -410,10 +410,9 @@ defmodule Telemetry.Metrics do
   @type tag :: term()
   @type tags :: [tag()]
   @type tag_values :: (:telemetry.event_metadata() -> :telemetry.event_metadata())
-  @type keep ::
+  @type keep_fun ::
           (:telemetry.event_metadata() -> boolean())
           | (:telemetry.event_metadata(), :telemetry.event_measurements() -> boolean())
-  @type drop :: (:telemetry.event_metadata() -> boolean())
   @type description :: nil | String.t()
   @type unit :: atom()
   @type time_unit_conversion() :: {time_unit(), time_unit()}
@@ -426,8 +425,8 @@ defmodule Telemetry.Metrics do
           | {:measurement, measurement()}
           | {:tags, tags()}
           | {:tag_values, tag_values()}
-          | {:keep, keep()}
-          | {:drop, drop()}
+          | {:keep, keep_fun()}
+          | {:drop, keep_fun()}
           | {:description, description()}
           | {:unit, unit() | time_unit_conversion() | byte_unit_conversion()}
           | {:reporter_options, reporter_options()}