Add OpenTracing documentation (#1345)

* Add OpenTracing documentation

Closes #274

* Reshuffle

* Update setup.md

dendrite-config.yaml

@@ -302,6 +302,8 @@ user_api:
     conn_max_lifetime: -1
 
 # Configuration for Opentracing.
+# See https://github.com/matrix-org/dendrite/tree/master/docs/tracing for information on
+# how this works and how to set it up.
 tracing:
   enabled: false
   jaeger:

docs/tracing/setup.md

@@ -0,0 +1,49 @@
+## OpenTracing Setup
+
+![Trace when sending an event into a room](/docs/tracing/jaeger.png)
+
+Dendrite uses [Jaeger](https://www.jaegertracing.io/) for tracing between microservices.
+Tracing shows the nesting of logical spans which provides visibility on how the microservices interact.
+This document explains how to set up Jaeger locally on a single machine.
+
+### Set up the Jaeger backend
+
+The [easiest way](https://www.jaegertracing.io/docs/1.18/getting-started/) is to use the all-in-one Docker image:
+```
+$ docker run -d --name jaeger \
+  -e COLLECTOR_ZIPKIN_HTTP_PORT=9411 \
+  -p 5775:5775/udp \
+  -p 6831:6831/udp \
+  -p 6832:6832/udp \
+  -p 5778:5778 \
+  -p 16686:16686 \
+  -p 14268:14268 \
+  -p 14250:14250 \
+  -p 9411:9411 \
+  jaegertracing/all-in-one:1.18
+```
+
+### Configuring Dendrite to talk to Jaeger
+
+Modify your config to look like: (this will send every single span to Jaeger which will be slow on large instances, but for local testing it's fine)
+```
+tracing:
+  enabled: true
+  jaeger:
+    serviceName: "dendrite"
+    disabled: false
+    rpc_metrics: true
+    tags: []
+    sampler:
+      type: const
+      param: 1
+```
+
+then run the monolith server with `--api true` to use polylith components which do tracing spans:
+```
+$ ./dendrite-monolith-server --tls-cert server.crt --tls-key server.key --config dendrite.yaml --api true
+```
+
+### Checking traces
+
+Visit http://localhost:16686 to see traces under `DendriteMonolith`.