[collectd 6] exec: add PUTMETRIC command

[collectd 6] exec: add PUTMETRIC command

Most existing setups using exec will use PUTVAL, which should just continue
to work with collectd 6 due to the plugin_dispatch_values compatibility
function.
New plugins should however use the new PUTMETRIC.
The respective command handler already exists. This commit just pipes it
through.

Signed-off-by: Leonard Göhrs <l.goehrs@pengutronix.de>