docs(metrics): align session metrics table columns

- Re-pad the session metrics table so the rows match the widened labels column

Author: appleboy

docs/METRICS.md

@@ -87,12 +87,12 @@ openssl rand -base64 48
 
 ### Session Metrics
 
-| Metric                     | Type      | Description             | Labels                                                |
-| -------------------------- | --------- | ----------------------- | ----------------------------------------------------- |
-| `sessions_active`          | Gauge     | Current active sessions | -                                                     |
-| `sessions_created_total`   | Counter   | Total sessions created  | -                                                     |
+| Metric                     | Type      | Description             | Labels                                                                  |
+| -------------------------- | --------- | ----------------------- | ----------------------------------------------------------------------- |
+| `sessions_active`          | Gauge     | Current active sessions | -                                                                       |
+| `sessions_created_total`   | Counter   | Total sessions created  | -                                                                       |
 | `sessions_expired_total`   | Counter   | Total sessions expired  | `reason` (logout, idle_timeout, fingerprint_mismatch, account_inactive) |
-| `session_duration_seconds` | Histogram | Session duration        | -                                                     |
+| `session_duration_seconds` | Histogram | Session duration        | -                                                                       |
 
 `sessions_active` increments on login and decrements on every **server-observable**
 session termination: explicit logout (`reason="logout"`), idle timeout