MINOR: promex: export "haproxy_sticktable_local_updates" metric

MINOR: promex: export "haproxy_sticktable_local_updates" metric

haproxy_sticktable_local_updates corresponds to the table->localupdate
counter, which is used internally by the peers protocol to identify
update messages in order to send and ack them among peers.

Here we decide to expose this information, as it is already the case in
"show peers" output, because it turns out that this value, which is
cumulative and grows in sync with the number of updates triggered on the
table due to changes initiated by the current process, can be used to
compute the update rate of the table. Computing the update rate of the
table (from the process point of view, ie: updates sent by the process and
not those received by the process), can be a great load indicator in order
to properly scale the infrastructure that is intended to handle the
table updates.

Note that there is a pitfall, which is that the value will eventually
wrap since it is stored using unsigned 32bits integer. Scripts or system
making use of this value must take wrapping into account between two
readings to properly compute the effective number of updates that were
performed between two readings. Also, they must ensure that the "polling"
rate between readings is small enough so that the value cannot wrap behind
their back.