Expose connection_string parameter into DevStack plugin

With this patch the user can select which OSProfiler driver to use
in DevStack. The configuration parameter is `OSPROFILER_CONNECTION_STRING`.
Default empty value refers to Ceilometer-based driver (value `messaging://`).

DevStack plugin's README is updated to explain new option.

Change-Id: I608cd3e7bafd29db72e9e3e4cb8eb7437cd81ab2

devstack/README.rst

@@ -5,24 +5,50 @@ Enabling OSProfiler using DevStack
 This directory contains the files necessary to run OpenStack with enabled
 OSProfiler in DevStack.
 
-To configure DevStack to enable OSProfiler edit
+OSProfiler has different drivers for trace processing. The default driver uses
-``${DEVSTACK_DIR}/local.conf`` file and add::
+Ceilometer to process and store trace events. Other drivers may connect
+to databases directly and do not require Ceilometer.
+
+To configure DevStack and enable OSProfiler edit ``${DEVSTACK_DIR}/local.conf``
+file and add the following to ``[[local|localrc]]`` section:
+
+  * to use specified driver::
+
+    enable_plugin osprofiler https://git.openstack.org/openstack/osprofiler master
+    OSPROFILER_CONNECTION_STRING=<connection string value>
+
+    the driver is chosen depending on the value of
+    ``OSPROFILER_CONNECTION_STRING`` variable (refer to the next section for
+    details)
+
+  * to use default Ceilometer driver::
 
     enable_plugin panko https://git.openstack.org/openstack/panko master
     enable_plugin ceilometer https://git.openstack.org/openstack/ceilometer master
     enable_plugin osprofiler https://git.openstack.org/openstack/osprofiler master
 
-to the ``[[local|localrc]]`` section.
+  .. note:: The order of enabling plugins matters.
 
-One can also configure a set of HMAC secrets, that are used for triggering of
+Run DevStack as normal::
-profiling in OpenStack services: only the requests that specify one of these
+
+    $ ./stack.sh
+
+
+Config variables
+----------------
+
+**OSPROFILER_HMAC_KEYS** - a set of HMAC secrets, that are used for triggering
+of profiling in OpenStack services: only the requests that specify one of these
 keys in HTTP headers will be profiled. E.g. multiple secrets are specified as
 a comma-separated list of string values::
 
     OSPROFILER_HMAC_KEYS=swordfish,foxtrot,charlie
 
-.. note:: The order of enabling plugins matter.
+**OSPROFILER_CONNECTION_STRING** - connection string to identify the driver.
+Default value is ``messaging://`` refers to Ceilometer driver. For a full
+list of drivers please refer to
+``http://git.openstack.org/cgit/openstack/osprofiler/tree/osprofiler/drivers``.
+Example: enable ElasticSearch driver with the server running on localhost::
 
-Run DevStack as normal::
+    OSPROFILER_CONNECTION_STRING=elasticsearch://127.0.0.1:9200
 
-    $ ./stack.sh

devstack/lib/osprofiler

@@ -53,6 +53,7 @@ function configure_osprofiler() {
             iniset $conf profiler enabled True
             iniset $conf profiler trace_sqlalchemy True
             iniset $conf profiler hmac_keys $OSPROFILER_HMAC_KEYS
+            iniset $conf profiler connection_string $OSPROFILER_CONNECTION_STRING
         fi
     done
     if [ -f $CEILOMETER_CONF ]

devstack/settings

@@ -4,5 +4,6 @@
 # of profiling in OpenStack services: profiling is only performed for
 # requests that specify one of these keys in HTTP headers.
 OSPROFILER_HMAC_KEYS=${OSPROFILER_HMAC_KEYS:-"SECRET_KEY"}
+OSPROFILER_CONNECTION_STRING=${OSPROFILER_CONNECTION_STRING:-"messaging://"}
 
 enable_service osprofiler