Opentracing Documentation (#5703)

* Opentracing survival guide

* Update decorator names in doc

* Doc cleanup

These are all alterations as a result of comments in #5703, it
includes mostly typos and clarifications. The most interesting
changes are:

- Split developer and user docs into two sections
- Add a high level description of OpenTracing

* newsfile

* Move contributer specific info to docstring.

* Sample config.

* Trailing whitespace.

* Update 5703.misc

* Apply suggestions from code review

Mostly just rewording parts of the docs for clarity.

Co-Authored-By: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

docs/opentracing.rst

@@ -0,0 +1,100 @@
+===========
+OpenTracing
+===========
+
+Background
+----------
+
+OpenTracing is a semi-standard being adopted by a number of distributed tracing
+platforms. It is a common api for facilitating vendor-agnostic tracing
+instrumentation. That is, we can use the OpenTracing api and select one of a
+number of tracer implementations to do the heavy lifting in the background.
+Our current selected implementation is Jaeger.
+
+OpenTracing is a tool which gives an insight into the causal relationship of
+work done in and between servers. The servers each track events and report them
+to a centralised server - in Synapse's case: Jaeger. The basic unit used to
+represent events is the span. The span roughly represents a single piece of work
+that was done and the time at which it occurred. A span can have child spans,
+meaning that the work of the child had to be completed for the parent span to
+complete, or it can have follow-on spans which represent work that is undertaken
+as a result of the parent but is not depended on by the parent to in order to
+finish.
+
+Since this is undertaken in a distributed environment a request to another
+server, such as an RPC or a simple GET, can be considered a span (a unit or
+work) for the local server. This causal link is what OpenTracing aims to
+capture and visualise. In order to do this metadata about the local server's
+span, i.e the 'span context', needs to be included with the request to the
+remote.
+
+It is up to the remote server to decide what it does with the spans
+it creates. This is called the sampling policy and it can be configured
+through Jaeger's settings.
+
+For OpenTracing concepts see 
+https://opentracing.io/docs/overview/what-is-tracing/.
+
+For more information about Jaeger's implementation see
+https://www.jaegertracing.io/docs/
+
+=====================
+Seting up OpenTracing
+=====================
+
+To receive OpenTracing spans, start up a Jaeger server. This can be done
+using docker like so:
+
+.. code-block:: bash
+
+   docker run -d --name jaeger
+     -p 6831:6831/udp \
+     -p 6832:6832/udp \
+     -p 5778:5778 \
+     -p 16686:16686 \
+     -p 14268:14268 \
+     jaegertracing/all-in-one:1.13
+
+Latest documentation is probably at
+https://www.jaegertracing.io/docs/1.13/getting-started/
+
+
+Enable OpenTracing in Synapse
+-----------------------------
+
+OpenTracing is not enabled by default. It must be enabled in the homeserver
+config by uncommenting the config options under ``opentracing`` as shown in
+the `sample config <./sample_config.yaml>`_. For example:
+
+.. code-block:: yaml
+
+  opentracing:
+    tracer_enabled: true
+    homeserver_whitelist:
+      - "mytrustedhomeserver.org"
+      - "*.myotherhomeservers.com"
+
+Homeserver whitelisting
+-----------------------
+
+The homeserver whitelist is configured using regular expressions. A list of regular
+expressions can be given and their union will be compared when propagating any
+spans contexts to another homeserver. 
+
+Though it's mostly safe to send and receive span contexts to and from
+untrusted users since span contexts are usually opaque ids it can lead to
+two problems, namely:
+
+- If the span context is marked as sampled by the sending homeserver the receiver will
+  sample it. Therefore two homeservers with wildly different sampling policies
+  could incur higher sampling counts than intended.
+- Sending servers can attach arbitrary data to spans, known as 'baggage'. For safety this has been disabled in Synapse
+  but that doesn't prevent another server sending you baggage which will be logged
+  to OpenTracing's logs.
+
+==================
+Configuring Jaeger
+==================
+
+Sampling strategies can be set as in this document:
+https://www.jaegertracing.io/docs/1.13/sampling/

docs/sample_config.yaml

@@ -1422,18 +1422,8 @@ opentracing:
     #enabled: true
 
     # The list of homeservers we wish to send and receive span contexts and span baggage.
-    #
-    # Though it's mostly safe to send and receive span contexts to and from
-    # untrusted users since span contexts are usually opaque ids it can lead to
-    # two problems, namely:
-    # - If the span context is marked as sampled by the sending homeserver the receiver will
-    # sample it. Therefore two homeservers with wildly disparaging sampling policies
-    # could incur higher sampling counts than intended.
-    # - Span baggage can be arbitrary data. For safety this has been disabled in synapse
-    # but that doesn't prevent another server sending you baggage which will be logged
-    # to opentracing logs.
-    #
-    # This a list of regexes which are matched against the server_name of the
+    # See docs/opentracing.rst
+    # This is a list of regexes which are matched against the server_name of the
     # homeserver.
     #
     # By defult, it is empty, so no servers are matched.