MINOR: reg-tests: add base prometheus test

MINOR: reg-tests: add base prometheus test

Add a base test to start with something, even though this is not
necessarily complete.
Also make use of the recent REQUIRE_SERVICE option to exclude it from
test list of it was not build with prometheus included.

note: I thought it was possible to send multiple requests within the
same client, but I'm getting "HTTP header is incomplete" from the second
request

Signed-off-by: William Dauchy <wdauchy@gmail.com>
(cherry picked from commit 76603f2552fafc23871a914600a18c3616959693)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>