Create metrics endpoints for general TMF-related information

Create endpoints:

• GET /tmf-api/metrics/registeredIndividuals
  • Exp. Resp: {registeredIndividuals: 23}
• GET /tmf-api/metrics/publishedServiceSpecifications
  • Exp. Resp: {publishedServiceSpecifications: 13} - TIP: get all ServiceCategories and count the allocated serviceCandidates
• GET /tmf-api/metrics/registeredResourceSpecifications
  • Exp. Resp: {registeredResourceSpecifications: 15}