fix: remove build tag from docker benchmark test, add changelog fragment

- Remove //go:build linux || darwin || windows from benchmark test —
  MockWatcherFactory is defined in the main test file which has no build
  constraint, so the linter could not resolve the symbol.
- Add changelog fragment for PR 49761.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: strawgate

changelog/fragments/1774755443-remove-unnecessary-allocations.yaml

@@ -0,0 +1,5 @@
+kind: enhancement
+summary: Remove unnecessary allocations in hot-path processors (docker metadata, kubernetes metadata, dissect, timestamp)
+component: libbeat
+pr:
+  - https://github.com/elastic/beats/pull/49761

libbeat/processors/add_docker_metadata/add_docker_metadata_benchmark_test.go

@@ -15,8 +15,6 @@
 // specific language governing permissions and limitations
 // under the License.
 
-//go:build linux || darwin || windows
-
 package add_docker_metadata
 
 import (