Reset gateway error on successful request

[michalpristas]

when gateway encounters error e.g dns name resolution

{"log.level":"warn","@timestamp":"2023-01-26T08:46:31.607Z","log.origin":{"file.name":"fleet/fleet_gateway.go","file.line":190},"message":"Possible transient error during checkin with fleet-server, retrying","error":{"message":"fail to checkin to fleet-server: all hosts failed: 1 error occurred:\n\t* requester 0/1 to host [https://<redacted>:443/](https:/<redacted>/) errored: Post \"[https://<redacted>/api/fleet/agents/7c2067ff-30ff-4628-b8cf-af36fa56a524/checkin](https://<redacted>/api/fleet/agents/7c2067ff-30ff-4628-b8cf-af36fa56a524/checkin)?\": lookup <redacted>: Temporary failure in name resolution\n\n"},"request_duration_ns":846590,"failed_checkins":2,"retry_after_ns":150942692753,"ecs.version":"1.6.0"}

it will report error to coordinator
this error is never reset so agent stays in failed state until restarted.

[michalpristas]

looks like this one was not properly migrated to v2: #1152
we should not report unhealthy due to fleet server connectivity issues as for #1148

that still stands right @cmacknz ?

[cmacknz]

Yes a single or a handful of network errors should not cause us to go unhealthy. There is logic in Fleet to detect that the agent is offline, we can rely on that to surface this problem.