etcdserver: Fix txn request 'took too long' warnings to use loggable request stringer

[jpbetz]

#9822 fixed log entries starting with "etcdserver: request", but not those starting with. "etcdserver: read-only range request":

etcdserver: read-only range request "compare:<target:VALUE key:\"a\" value:\"1\" > success:<request_range:<key:\"/a\" range_end:\"/b\" > > failure:<request_range:<key:\"/a\" range_end:\"/b\" > > " with result "responses:<range_response_count:0> size:12" took too long (48.373µs) to execute

This fixes the logger to handle the "read-only range" warnings correctly:

etcdserver: read-only range request "compare:<target:VALUE key:\"a\" value_size:1 > success:<request_range:<key:\"/a\" range_end:\"/b\" > > failure:<request_range:<key:\"/a\" range_end:\"/b\" > >" with result "responses:<range_response_count:0> size:12" took too long (37.32µs) to execute

[gyuho]

lgtm thanks!