Support enabling opentracing by user (#9978)

Add a config option which allows enabling opentracing by user id, eg for
debugging requests made by a test user.

docs/opentracing.md

@@ -42,17 +42,17 @@ To receive OpenTracing spans, start up a Jaeger server. This can be done
 using docker like so:
 
 ```sh
-docker run -d --name jaeger
+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
+  jaegertracing/all-in-one:1
 ```
 
 Latest documentation is probably at
-<https://www.jaegertracing.io/docs/1.13/getting-started/>
+https://www.jaegertracing.io/docs/latest/getting-started.
 
 ## Enable OpenTracing in Synapse
 
@@ -62,7 +62,7 @@ as shown in the [sample config](./sample_config.yaml). For example:
 
 ```yaml
 opentracing:
-  tracer_enabled: true
+  enabled: true
   homeserver_whitelist:
     - "mytrustedhomeserver.org"
     - "*.myotherhomeservers.com"
@@ -90,4 +90,4 @@ to two problems, namely:
 ## Configuring Jaeger
 
 Sampling strategies can be set as in this document:
-<https://www.jaegertracing.io/docs/1.13/sampling/>
+<https://www.jaegertracing.io/docs/latest/sampling/>.

docs/sample_config.yaml

@@ -2845,7 +2845,8 @@ opentracing:
     #enabled: true
 
     # The list of homeservers we wish to send and receive span contexts and span baggage.
-    # See docs/opentracing.rst
+    # See docs/opentracing.rst.
+    #
     # This is a list of regexes which are matched against the server_name of the
     # homeserver.
     #
@@ -2854,19 +2855,26 @@ opentracing:
     #homeserver_whitelist:
     #  - ".*"
 
+    # A list of the matrix IDs of users whose requests will always be traced,
+    # even if the tracing system would otherwise drop the traces due to
+    # probabilistic sampling.
+    #
+    # By default, the list is empty.
+    #
+    #force_tracing_for_users:
+    #  - "@user1:server_name"
+    #  - "@user2:server_name"
+
     # Jaeger can be configured to sample traces at different rates.
     # All configuration options provided by Jaeger can be set here.
-    # Jaeger's configuration mostly related to trace sampling which
+    # Jaeger's configuration is mostly related to trace sampling which
     # is documented here:
-    # https://www.jaegertracing.io/docs/1.13/sampling/.
+    # https://www.jaegertracing.io/docs/latest/sampling/.
     #
     #jaeger_config:
     #  sampler:
     #    type: const
     #    param: 1
-
-    #  Logging whether spans were started and reported
-    #
     #  logging:
     #    false