Move HTTP example for Trace into trace directory

[ymotongpoo]

The intention here is to add metrics directory for Cloud Monitoring samples and to make example directory as the entire root of all samples.

Planned structure is as the following:

example
├── metrics
│   ├── go.mod
│   ├── go.sum
│   └── main.go --> simple metric sample
└── trace
    └── http --> simple distributed trace over HTTP sample (client and server)
        ├── client
        │   └── client.go
        ├── go.mod
        ├── go.sum
        └── server
            ├── modd.conf
            └── server.go