Fix kubernetes host:port relabel regex.
Fix kubernetes host:port relabel regex.

This change corrects a bug introduced by PR
https://github.com/m-lab/prometheus-support/pull/3

The regex uses three groups: the hostname, an optional port, and the
prefered port from a kubernetes annotation.

Previously, the second group should have been ignored if a :port was not
present in the input. However, making the port group optional with the
"?" had the unintended side-effect of allowing the hostname regex "(.+)"
to match greedily, which included the ":port" patterns up to the ";"
separating the hostname from the kubernetes port annotation.

This change updates the regex for the hostname to match any non-":"
characters. This forces the regex to stop if a ":port" is present and
allow the second group to match the optional port.

stephen-soltesz/prometheus-support,stephen-soltesz/prometheus-support,stephen-soltesz/prometheus-support