Re-add jaeger-tracing

documentation/en/jaeger-tracing.md

@@ -0,0 +1,26 @@
+# Jaeger Tracing
+
+Lotus has tracing built into many of its internals. To view the traces, first download [Jaeger](https://www.jaegertracing.io/download/) (Choose the 'all-in-one' binary). Then run it somewhere, start up the lotus daemon, and open up localhost:16686 in your browser.
+
+## Open Census
+
+Lotus uses [OpenCensus](https://opencensus.io/) for tracing application flow. This generates spans through the execution of annotated code paths.
+
+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](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()
+```