The TestSessionsMetrics test from connection_test.go:214 is flaky

It can be hard to replicate, but it happens semi frequently in main (at least 7cd080ae):

--- FAIL: TestSessionsMetrics (0.00s)
    connection_test.go:218:
        	Error Trace:	/Users/ash/src/gitlab/gitlab-development-kit/gitlab-shell/internal/sshd/connection_test.go:214
        	Error:      	Max difference between 1 and 0 allowed is 0.1, but difference was 1
        	Test:       	TestSessionsMetrics