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

Signed-off-by: Edwin Kempin <d43fb6c98c97e553b671ed9a9318100f1de386e7@google.com>
Change-Id: I992faceae4e3a6261f2f54f36d4bac1e23724522

GerritCodeReview/plugins_code-owners,GerritCodeReview/plugins_code-owners,GerritCodeReview/plugins_code-owners,GerritCodeReview/plugins_code-owners