Add a changelog entry for veneur-emit stderr/stdout streaming
* `veneur-emit` now takes `-span_service`, `-trace_id`, `-parent_id`, and `-indicator` arguments. These (combined with `-ssf`) allow submitting spans for tracing when recording timing data for commands. In addition, `-timing` with `-ssf` now works, too.
* The `github.com/stripe/veneur/ssf` package now has a few helper functions to create samples that can be attached to spans: `Count`, `Gauge`, `Histogram`, `Timing`. In addition, veneur now has a span sink that extracts these samples and treats them as metrics. Thanks, [antifuchs](https://github.com/antifuchs)
* Spans sent to Lightstep now have an `indicator` tag set, indicating whether the span is an indicator span or not. Thanks, [aditya](https://github.com/chimeracoder)!
+* `veneur-emit -command` now streams output from the invoked program's stdout/stderr to its own stdout/stderr. Thanks, [antifuchs](https://github.com/antifuchs)!
## Improvements
* Veneur now emits a timer metric giving the duration (in nanoseconds) of every "indicator" span that it receives, if you configure the setting `indicator_span_timer_name`. Thanks, [antifuchs](https://github.com/antifuchs)!
