Update env_vars metrics name.

Bug: b/265929888
Change-Id: Iaf2fd1d509f4390d26c8f4f74f863bbe3551600c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4219773
Reviewed-by: Gavin Mak <gavinmak@google.com>
Auto-Submit: Joanna Wang <jojwang@chromium.org>
Commit-Queue: Gavin Mak <gavinmak@google.com>

metrics.py

@@ -230,7 +230,7 @@ class MetricsCollector(object):
     self.add('command', command_name)
     for env in DEPOT_TOOLS_ENV:
       if env in os.environ:
-        self.add_repeated('env_variables', {
+        self.add_repeated('env_vars', {
             'name': env,
             'value': os.environ.get(env)
         })