Add extra information about logging to GLQL troubleshooting guide

Alisa Frunzarequested to merge
glql-ga-runbooks into master

I was thinking that we do not log data properly for failed GLQL requests and spent some time investigating it. It turned out I was confused in log structure and we do have all required data logged (see related comment for more details). This MR updates the Logging section in GLQL troubleshooting guide to add more details on how the logs are structured and how to find info specifically for failed requests.

Relates to gitlab-org/gitlab#550624 (closed).

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