Previously the %{} syntax has always meant fetching a field value from an event.

This change introduces something else, where %{metric} is a special case allowing you get the computed metric name.

To avoid that special-casing, can we implement it slightly differently? Would a 'namespace' concept be sufficient? The statsd output uses it to prefix (I think?) all metrics emitted. I imagine your 'foo.bar' above could simply be 'namespace => "foo.bar"' or something.

Is a suffix something you need?  If we need prefix/suffix, we can do two settings: metrics_prefix and metrics_suffix - thoughts?