Add timeout option to haproxy input

[danielnelson]

closes #7458

Required for all PRs:

• Signed CLA.
• Associated README.md updated.
• Has appropriate unit tests.

[reimda]

I don't think this approach will fix the problem in the issue. The timeouts in the issue are for socket unix connections but these changes are for HTTP connections. Sockets go through gatherServerSocket so these changes won't have any effect.

Actually, I don't understand why the input doesn't already return partial responses. The Gather func loops over the endpoints (the sockets) and gathers each in its own goroutine. It seems like this should add metrics for the sockets that are responding even if some are not responding. There's something else going on here.

If the problem is around connecting to the socket or writing to it, the user should be getting errors from telegraf. It seems more likely that when the haproxy process isn't responding it doesn't accept connections which should log an error "Could not connect to socket". There's also a chance it's accepting connection but not accepting write so they'd see "Could not write to socket".

If it's a write problem, Maybe we need to add a deadline on the connection in gatherServerSocket. After dial returns

err := c.SetDeadline(time.Now().Add(4*time.Second))

but we should confirm that they see "Could not write to socket" in their logs before adding that fix.

Also, once it's more clear what the failure is, we ought to have a unit test for the workaround. haproxy_test.go already does the work to stub/mock haproxy expected response so a new unit test won't be too much trouble. We can copy most of TestHaproxyGeneratesMetricsUsingSocket but add a new handler mock that doesn't accept connections or writes or whatever, along with the existing mocks of expected response.