Create metrics endpoints for NFV-related information

Create endpoints:

• GET /osapi/metrics/registeredNsds (currently appearing as "experiments")
  • Exp. Resp: { registeredNsds: 32 }
  • existing endpoints: /osapi/experiments, /osapi/admin/experiments
• GET /osapi/metrics/registeredVnfs (currently appearing as "vxfs")
  • Exp. Resp: { registeredVnfs: 45 }
  • existing endpoint: /osapi/admin/vxfs
• GET /osapi/metrics/registeredManoProviders
  • Exp. Resp: { registeredManoProviders: 3 }
  • existing endpoint: /osapi/admin/manoproviders