DOC: Add maintainers for the Prometheus exporter

DOC: Add maintainers for the Prometheus exporter

William Dauchy and Christopher Faulet are the official maintainers of the
Prometheus exporter. William better knows the Prometheus usages and is the
referent for the features while Christopher's role is more code-centric, he
takes care of bugs and the exporter integration into Haproxy.

(cherry picked from commit 84c88a58c5153b8fffa509934375c92435298a1b)
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit 650f1f491d0346640cfdbc810705767d24b11189)
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit d76c981cd74b78d62131d25b23bce010287ce821)
Signed-off-by: Willy Tarreau <w@1wt.eu>