Send system.process.cpu.start_time as a field

[lahsivjar]

• Converts the system.process.cpu.start_time to a field with string date. The string date format allows the field to be correctly detected as a date field rather than an int field by the dynamic mapping used by APM. The date mapping is required by Kibana to parse the field correctly in the Process tab.
• Adds event.dataset for the process summary metric which is again required by the Process tab.