[ingress] Parse the value of delaySec query param

[igalshilman]

This PR fixes a bug, where we previously tried parsing the queryParam name (key) instead of its value.

Before:

curl http://localhost:8080/greeter/greet/send?delaySec=1
{"message":"bad delaySec query parameter, must be a number: ParseIntError { kind: InvalidDigit }"}

After:

{
  invocationId: 'inv_1eNu2L3WKoEZ332IKyt2c6xNzg4HYl0SeR',
  executionTime: '2024-05-06T07:57:09.777000000Z'
}