Update docs for database load balancing logs

The database load balancing docs had some mention of the logs that
needed to be updated, and the logs doc had an absolute link to
gitlab.com instead of a relative link to a markdown file.

doc/administration/database_load_balancing.md

@@ -212,28 +212,25 @@ without it immediately leading to errors being presented to the users.
 
 ## Logging
 
-The load balancer logs various messages, such as:
+The load balancer logs various events in
+[`database_load_balancing.log`](logs.md#database_load_balancinglog-premium-only), such as
 
 - When a host is marked as offline
 - When a host comes back online
 - When all secondaries are offline
+- When a read is retried on a different host due to a query conflict
 
-Each log message contains the tag `[DB-LB]` to make searching/filtering of such
-log entries easier. For example:
+The log is structured with each entry a JSON object containing at least:
 
-```
-[DB-LB] Host 10.123.2.5 came back online
-[DB-LB] Marking host 10.123.2.7 as offline
-[DB-LB] Marking host 10.123.2.7 as offline
-[DB-LB] Marking host 10.123.2.7 as offline
-[DB-LB] Marking host 10.123.2.7 as offline
-[DB-LB] Marking host 10.123.2.7 as offline
-[DB-LB] Host 10.123.2.6 came back online
-[DB-LB] Marking host 10.123.2.7 as offline
-[DB-LB] Marking host 10.123.2.7 as offline
-[DB-LB] Marking host 10.123.2.7 as offline
-[DB-LB] Host 10.123.2.7 came back online
-[DB-LB] Host 10.123.2.7 came back online
+- An `event` field useful for filtering.
+- A human-readable `message` field.
+- Some event-specific metadata. For example, `db_host`
+- Contextual information that is always logged. For example, `severity` and `time`.
+
+For example:
+
+```json
+{"severity":"INFO","time":"2019-09-02T12:12:01.728Z","correlation_id":"abcdefg","event":"host_online","message":"Host came back online","db_host":"111.222.333.444","db_port":null,"tag":"rails.database_load_balancing","environment":"production","hostname":"web-example-1","fqdn":"gitlab.example.com","path":null,"params":null}
 ```
 
 ## Handling Stale Reads

doc/administration/logs.md

@@ -339,11 +339,11 @@ installations from source.
 [Rack Attack]: ../security/rack_attack.md
 [Rate Limit]: ../user/admin_area/settings/rate_limits_on_raw_endpoints.md
 
-## `database_load_balancing.log`
+## `database_load_balancing.log` **(PREMIUM ONLY)**
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/15442) in GitLab 12.3.
 
-This log is used for observability of [Database Load Balancing](database_load_balancing.md).
+Contains details of GitLab's [Database Load Balancing](database_load_balancing.md).
 It is stored at:
 
 - `/var/log/gitlab/gitlab-rails/database_load_balancing.log` for Omnibus GitLab packages.