[logging] log messages show remoteAddress in userAgent

[koshatul]

Kibana version: 7.9.0

Original install method (e.g. download page, yum, from source, etc.): apt

Describe the bug: Log messages are showing the remoteAddress in the userAgent field.

Steps to reproduce:

1. Setup Kibana
2. Access Kibana
3. Check Logs

Expected behavior: probably replicate the request.userAgent into the userAgent field.

Provide logs and/or server output (if relevant):

Sep 02 14:18:34 kibana-test kibana[1121]: {"type":"response","@timestamp":"2020-09-02T04:18:34Z","tags":[],"pid":1121,"method":"get","statusCode":200,"req":{"url":"/ui/favicons/manifest.json","method":"get","headers":{"host":"kibana-test:8080","connection":"keep-alive","user-agent":"Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/84.0.4147.135 Safari/537.36","accept":"*/*","sec-fetch-site":"same-origin","sec-fetch-mode":"cors","sec-fetch-dest":"empty","referer":"https://kibana-test:8080/spaces/space_selector","accept-encoding":"gzip, deflate, br","accept-language":"en-US,en;q=0.9"},"remoteAddress":"192.168.49.2","userAgent":"192.168.49.2","referer":"https://kibana-test:8080/spaces/space_selector"},"res":{"statusCode":200,"responseTime":4,"contentLength":9},"message":"GET /ui/favicons/manifest.json 200 4ms - 9.0B"}

Any additional context:
in src/legacy/server/logging/log_format.js#L94:

      data.req = {
        url: event.path,
        method: event.method || '',
        headers: event.headers,
        remoteAddress: source.remoteAddress,
        userAgent: source.remoteAddress,
        referer: source.referer,
      };

[elasticmachine]

Pinging @elastic/kibana-platform (Team:Platform)