Clean up opentracing configuration options (#5712)

Clean up config settings and dead code.

This is mostly about cleaning up the config format, to bring it into line with our conventions. In particular:
 * There should be a blank line after `## Section ##' headings
 * There should be a blank line between each config setting
 * There should be a `#`-only line between a comment and the setting it describes
 * We don't really do the `#  #` style commenting-out of whole sections if we can help it
 * rename `tracer_enabled` to `enabled`

While we're here, do more config parsing upfront, which makes it easier to use
later on.

Also removes redundant code from LogContextScopeManager.

Also changes the changelog fragment to a `feature` - it's exciting!

docs/sample_config.yaml

@@ -1409,17 +1409,34 @@ password_config:
 
 
 ## Opentracing ##
-# These settings enable opentracing which implements distributed tracing
-# This allows you to observe the causal chain of events across servers
-# including requests, key lookups etc. across any server running
-# synapse or any other other services which supports opentracing.
-# (specifically those implemented with jaeger)
 
-#opentracing:
-#  # Enable / disable tracer
-#  tracer_enabled: false
-#  # The list of homeservers we wish to expose our current traces to.
-#  # The list is a list of regexes which are matched against the
-#  # servername of the homeserver
-#  homeserver_whitelist:
-#    - ".*"
+# These settings enable opentracing, which implements distributed tracing.
+# This allows you to observe the causal chains of events across servers
+# including requests, key lookups etc., across any server running
+# synapse or any other other services which supports opentracing
+# (specifically those implemented with Jaeger).
+#
+opentracing:
+    # tracing is disabled by default. Uncomment the following line to enable it.
+    #
+    #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
+    # homeserver.
+    #
+    # By defult, it is empty, so no servers are matched.
+    #
+    #homeserver_whitelist:
+    #  - ".*"