Add support for timeout offset

[mikael-lindstrom]

Allow the MongoDB timeout to be configured relative to the Prometheus scrape timeout. This ensures the Prometheus scrape will get a response even if the exporter cannot connect to MongoDB. Implementation is based on how its done in the blackbox_exporter. This in combination with #624 makes the mongodb_exporter return mongodb_up 0 when it cannot connect to MongoDB.

I have not been able to verify that TestGetEncryptionInfo pass since I cannot get percona/percona-server-mongodb:5.0.13-1 to start on my M1 Mac (regardless if I use docker-compose or start the image directly). All other tests pass.

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• Tests passed.
• Fix conflicts with target branch.
• Update jira ticket description if needed.
• Attach screenshots/console output to confirm new behavior to jira ticket, if applicable.

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[it-percona-cla]

All committers have signed the CLA.

[BupycHuk]

@mikael-lindstrom could you please update your branch?

[mikael-lindstrom]

@BupycHuk Updated the branch now 👍

[JiriCtvrtka]

@mikael-lindstrom Looks good. Could you also add check when TimeoutOffset is 0 or less? And just show error message. Thank you.

[mikael-lindstrom]

I resolved the conflicts and added a check for TimeoutOffset being greater than 0. I also changed the name for the flag from mongodb.timeout-offset to web.timeout-offset since it's not directly related to MongoDB and it was a bit confusing with the new mongodb.connect-timeout-ms flag.

I pushed the latest commit 6 hours ago but for some reason this PR shows "Processing updates". Not sure why but the changes are in my branch.

[JiriCtvrtka]

@mikael-lindstrom Could you try to push it once more? Maybe try add just empty commit. Thank you.

[mikael-lindstrom]

@JiriCtvrtka Seems to have worked with an empty commit 👍

[JiriCtvrtka]

Great job, thank you.