Search backend: add stream tracing

Created by: camdencheek

This adds stream tracing to all our jobs. For each event sent down the stream, we log the number of results in the event. This is useful for debugging latency and visualizing the frequency of events across the job tree. Additionally, we log the total number of events streamed by each job.

It's possible that on sourcegraph.com, this will generate traces that are unreasonably large. There are a few options from there: including batching events, only logging first and last event, or just disabling logging events on the trace. I don't expect issues, so I don't plan to prematurely optimize here.

We may also find that this information is not valuable, or too noisy to be useful. I'm mostly just experimenting, so feel free to riff.

Test plan

Manually tested. See screenshot: