Hello Bhupesh
In the topology that you have shared, there is a core Provider network that belongs to the ISP through which you are connecting your sites. Such a network is often based on MPLS. MPLS networks have Provider (P ) routers, which are devices internal to the core network, and Provider Edge (PE) routers, which are the devices via which the customers connect to the network.
An ISP will have PEs in various areas, but a customer may not be physically close enough to reach that device directly. An intermediate network may be necessary to allow a customer to connect to the PE. This is where Metro Ethernet is often used.
A Metro Ethernet (ME) network may be connected to one or more PEs of an ISP. The ME network will be used to connect multiple customers to the PEs of the ISP, so the configuration of VLANs within the ME network is necessary. This can be done in many ways and depends on the number of customers, and the requirements of the ISP. Some documentation and lessons that will help you in this are the following:
Because the topic you are touching upon is so vast, I hope that this will give you a beginning in further studying and understanding all of the concepts involved with ME.
I hope this has been helpful!
Laz