Support request tracing for REST calls by setting a header in the request

At the moment request tracing for REST calls is enabled by setting the
'trace' or 'trace=<trace-id>' request parameter. This is good for manual
use since adding the request parameter to the URL is very easy and can
be done quickly.

For providing copy-pastable examples for how to do tracing this is not
ideal, since it depends on the concrete URL how the request parameter
would need to be set, e.g. it can be that '?trace' or '&trace' needs to
be appended depending on whether the URL already contains request
parameters or not.

With this change we now support enabling request tracing for REST calls
also by setting a 'X-Gerrit-Trace' header in the request. For manual use
this is less easy but it makes providing copy-pastable examples for how
to do tracing easier as one can now do:

  curl -D /tmp/gerrit -H X-Gerrit-Trace URL
  grep X-Gerrit-Trace /tmp/gerrit

Change-Id: I793ca9fff83ef23f5720390931599a9a85e868c7
Signed-off-by: Edwin Kempin <ekempin@google.com>

Documentation/rest-api.txt

diff --git a/Documentation/rest-api.txt b/Documentation/rest-api.txt
index 97cca38..8f6a47b 100644
--- a/Documentation/rest-api.txt
+++ b/Documentation/rest-api.txt
@@ -210,6 +210,15 @@
   GET /changes/myProject~master~I8473b95934b5732ac55d26311a706c9c2bde9940/suggest_reviewers?trace&q=J
 ----
 
+Alternatively request tracing can also be enabled by setting the
+`X-Gerrit-Trace` header:
+
+.Example Request
+----
+  GET /changes/myProject~master~I8473b95934b5732ac55d26311a706c9c2bde9940/suggest_reviewers?q=J
+  X-Gerrit-Trace: issue/123
+----
+
 Enabling tracing results in additional logs with debug information that
 are written to the `error_log`. All logs that correspond to the traced
 request are associated with the trace ID. The trace ID is returned with