Update BBS metrics in docs.
* The BBS Request Latency is emitted periodically.
* Open File Descriptors is now available as a metric.
[finishes
| `LockHeld.` `v1-locks-bbs_lock` | Whether a BBS holds the BBS lock (in consul): 1 means the lock is held, and 0 means the lock was lost. Emitted periodically by the active BBS server. | 0 or 1 (boolean) |
| `LockHeldDuration.` `v1-locks-bbs_lock` | Time the active BBS has held the BBS lock (in consul). Emitted periodically by the active BBS server. | ns |
| `MigrationDuration` | Time the BBS took to run migrations against its persistence store. Emitted each time a BBS becomes the active master. | ns |
+|`OpenFileDescriptors` | Current (non-cumulative) number of open file descriptors held by the BBS. Emitted periodically. | number |
| `RequestCount` | Cumulative number of requests the BBS has handled through its API. Emitted periodically. | number |
-| `RequestLatency` | Time the BBS took to handle requests to its API endpoints. Emitted when the BBS API handles requests. | ns |
+| `RequestLatency` | Time the BBS took to handle requests to its API endpoints. Emitted periodically. | ns |
| `TasksCompleted` | Total number of Tasks that have completed. Emitted periodically. | number |
| `TasksPending` | Total number of Tasks that have not yet been placed on a cell. Emitted periodically. | number |
| `TasksResolving` | Total number of Tasks locked for deletion. Emitted periodically. | number |
