Limit in-flight Prometheus metric queries and timeout slow requests in Praefect

Praefect currently doesn't limit the number of in-flight metric queries nor does it assign a timeout to them. We should set a limit for in-flight queries and timeouts for both the HTTP request and the collectors.

Related to: gitlab-com/gl-infra/production#2918 (closed)

Edited Nov 11, 2020 by Sami Hiltunen

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