improve tracing doc

docs/tracing.md

@@ -1,9 +1,30 @@
 ## Tracing
 
-Lotus uses Jaeger for tracing. Currently it always uses
-localhost and default port (`localhost:6831`).
+Lotus uses [OpenCensus](https://opencensus.io/) for tracing application flow.
+This generates spans
+through the execution of annotated code paths.
 
-During dev you can use `jaeger-all-in-one` from: https://www.jaegertracing.io/download/
-Start the `jaeger-all-in-one` and open http://localhost:16686/ to view traces.
-j
-In production: you tell me and I might WTFM.
+Currently it is set up to use jaeger, though other tracing backends should be
+fairly easy to swap in.
+
+## Running Locally
+
+To easily run and view tracing locally, first, install jaeger. The easiest way
+to do this is to download the binaries from
+https://www.jaegertracing.io/download/ and then run the `jaeger-all-in-one`
+binary. This will start up jaeger, listen for spans on `localhost:6831`, and
+expose a web UI for viewing traces on `http://localhost:16686/`.
+
+Now, to start sending traces from lotus to jaeger, set the environment variable
+`LOTUS_JAEGER` to `localhost:6831`, and start the `lotus daemon`.
+
+Now, to view any generated traces, open up `http://localhost:16686/` in your
+browser.
+
+## Adding Spans
+To annotate a new codepath with spans, add the following lines to the top of the function you wish to trace:
+
+```go
+ctx, span := trace.StartSpan(ctx, "put function name here")
+defer span.End()
+```