Add HAProxy timer metrics

[warnerpr]

There are useful to see backend performance. They are new in HAProxy 1.5, but the collector should still work with older versions.

From http://cbonte.github.io/haproxy-dconv/configuration-1.5.html#9.1:

 58. qtime [..BS]: the average queue time in ms over the 1024 last requests
 59. ctime [..BS]: the average connect time in ms over the 1024 last requests
 60. rtime [..BS]: the average response time in ms over the 1024 last requests
     (0 for TCP)
 61. ttime [..BS]: the average total session time in ms over the 1024 last
     requests

[LeoCavaille]

Thanks @warnerpr for your contribution!
We're working on fixing the CI, but the sub-job of HAproxy (which is running 1.5 for integration testing) on python 2.7 seem to have succeeded which is the most important 😉
Let's do a final review and merge this for 5.4.

[LeoCavaille]

@warnerpr reviewed and merged your changes in a rebased version:

• bbc3412
  I also added on top c6eee4f to add more versions of HAProxy integration testing in our CI, to ensure non-regression for the future.
  Thanks again!

[warnerpr]

Nice. Thanks @LeoCavaille. Great idea to test against the older versions.