metrics.md: Document the prefix that is used for all metrics

Signed-off-by: Edwin Kempin <ekempin@google.com>
Change-Id: I992faceae4e3a6261f2f54f36d4bac1e23724522
diff --git a/resources/Documentation/metrics.md b/resources/Documentation/metrics.md
index 5cff902..0fc2673 100644
--- a/resources/Documentation/metrics.md
+++ b/resources/Documentation/metrics.md
@@ -3,6 +3,8 @@
 The @PLUGIN@ plugin exports several metrics which can give insights into the
 usage and performance of the code owners functionality.
 
+All metrics have the following prefix: `plugins/@PLUGIN@/`
+
 ## <a id="latencyMetrics"> Latency Metrics
 
 * `add_change_message_on_add_reviewer`: