EventService: Fix type mismatch in MetricReport

Fix the type mismatch in MetricReport data which
is sent to Event Subscribers. Change below properties
type to match with MetricReport schema.
 - Timestamp: It should be uint32_t type.
 - MetricValue: It should be string type.
 
 Tested:
  - Create MetricReport subscription and report contains
    correct data types for Timestamp and MetricValue.

Signed-off-by: Wludzik, Jozef <jozef.wludzik@intel.com>
Signed-off-by: AppaRao Puli <apparao.puli@linux.intel.com>
Change-Id: I0a52b6963e7bedda89a216256f64764cd8799bf1