riemann: Add support for tagging the outgoing events

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
riemann: Unless tags() are explicitly set, send the message tags

If no tags() are configured, use the tags already set on the message by
default.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
riemann: Add support for setting attributes

Riemann supports setting custom attributes, add support for that, based
on value-pairs. We use the attributes() keyword for clarity, instead of
value-pairs(), since attributes are auxillary data.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
riemann: Enable flow-control in the example config

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>