feat: log REST request headers

[vchudnov-g]

Fixes #1494

Note that the REST server logs metadata unconditionally, so logging the request headers is not gated on passing -v to Showcase. Thus, to show headers for both the gRPC and REST transports, gapic-showcase run -v will work.

Sample output on the server console:

2024/04/11 10:15:43 Received POST request matching '/v1beta1/echo:wait': "/v1beta1/echo:wait"
2024/04/11 10:15:43   urlPathParams (expect 0, have 0): map[]
2024/04/11 10:15:43   urlRequestHeaders:
    Content-Length: "50"
    User-Agent: "curl/8.5.0"
    Accept: "*/*"
    X-Goog-Api-Client: "rest/0.0.0 gapic/0.0.0"
    Content-Type: "application/json"
2024/04/11 10:15:43   request: {
  "ttl": "3s",
  "success": {
    "content": "rainy june"
  }
}