Make Jaeger fully configurable (#5694)

* Allow Jaeger to be configured

* Update sample config

docs/sample_config.yaml

@@ -1430,3 +1430,19 @@ opentracing:
     #
     #homeserver_whitelist:
     #  - ".*"
+
+    # 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
+    # is documented here:
+    # https://www.jaegertracing.io/docs/1.13/sampling/.
+    #
+    #jaeger_config:
+    #  sampler:
+    #    type: const
+    #    param: 1
+
+    #  Logging whether spans were started and reported
+    #
+    #  logging:
+    #    false