Add check-xml pre-commit

The driving use case is to validate telemetry xml metadata configuration
files, all found under tools/metrics. These are the files used to
propagate metadata about the metrics inside Google.

b/296056775

Change-Id: I511a4311b0ce931e7ab3dc945c7b24f3f1a2dc7a

.pre-commit-config.yaml

@@ -27,7 +27,7 @@ exclude: |
             testing|
             third_party|
             tools/gyp|
-            tools/metrics|
+            tools/metrics/(.*\.(?!xml$)[^/]*|[^/.]+)$|
             tools/python|
             url
         )/
@@ -49,6 +49,7 @@ repos:
     -   id: end-of-file-fixer
     -   id: trailing-whitespace
     -   id: mixed-line-ending
+    -   id: check-xml
 
 -   repo: https://cobalt.googlesource.com/codespell
     rev: 67c489d36dd4c52cbb9e4755d90c35c6231842ef  # v2.0.0