Add secondary lag message on Git push over HTTP

What does this MR do?

Adds the remote: Current replication lag: X seconds in a push over HTTP to a secondary:

$ git push
[master b7e3005] Update README.md
 1 file changed, 1 insertion(+)
warning: redirecting to http://localhost:3001/-/push_from_secondary/4/twitter/typeahead-js.git/
Enumerating objects: 5, done.
Counting objects: 100% (5/5), done.
Delta compression using up to 8 threads
Compressing objects: 100% (3/3), done.
Writing objects: 100% (3/3), 969 bytes | 969.00 KiB/s, done.
Total 3 (delta 2), reused 0 (delta 0)
remote: 
remote: Current replication lag: 9 seconds
remote: 
To http://localhost:3002/twitter/typeahead-js.git
   1e8b0c6..b7e3005  master -> master

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/9195

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team