Added client ID env var

src/fixit/engine.py

@@ -4,6 +4,7 @@
 # LICENSE file in the root directory of this source tree.
 
 import logging
+import os
 import time
 from collections import defaultdict
 from contextlib import contextmanager
@@ -65,9 +66,17 @@ def collect_violations(
 
         The optional `metrics_hook` parameter will be called (if provided) after all
         lint rules have finished running, passing in a dictionary of
-        ``RuleName.visit_function_name`` -> ``duration in microseconds``.
+        {
+            CLIENT_ID: an optional client id set by environment variable,
+            Duration.RuleName.visit_function_name: duration in microseconds,
+            ViolationCount.RuleName: number of violations,
+            ViolationCountReplacement.RuleName: number of violations with replacements,
+            ViolationCount.Total: total number of violations,
+        }
         """
 
+        self.metrics["CLIENT_ID"] = os.environ.get("FIXIT_CLIENT_ID", "default")
+
         @contextmanager
         def visit_hook(name: str) -> Iterator[None]:
             start = time.perf_counter()