Provide explicit help string of resource-metadata

Currently, the "--resource-metadata" in ceilometer sample-create command
is not easy to use. For compatibility, this patch just provides an
explicit help string for the parameter.

Change-Id: I6d68d5b653c3b33808cba6f9a781301fba9b44e3
Closes-Bug: #1324759

ceilometerclient/v2/shell.py

@@ -144,7 +144,8 @@ def do_sample_list(cc, args):
 @utils.arg('--sample-volume', metavar='<SAMPLE_VOLUME>', required=True,
            help='The sample volume.')
 @utils.arg('--resource-metadata', metavar='<RESOURCE_METADATA>',
-           help='Resource metadata.')
+           help='Resource metadata. Provided value should be a set of '
+                'key-value pairs e.g. {"key":"value"}.')
 @utils.arg('--timestamp', metavar='<TIMESTAMP>',
            help='The sample timestamp.')
 def do_sample_create(cc, args={}):