feat: Optional EVENT_INGRESS_URI

[yagihash]

WHAT

• Make app not use CloudEvents client when METRICS is set to false

WHY

CloudEvents logging is not essential to run the service. This is for who wanna run the service with minimum requirements. And ceclient is not used when METRICS is set to false in fact.

ISSUE

#949

---

Since it is an update on main, I did not add any test.
Instead of that, I have quickly checked the behavior with those commands below.
This should not break anything since CloudEvents client is referred only when METRICS is set to true.

METRICS=false \
PORT=8080 \
GITHUB_APP_ID=11111 \
STS_DOMAIN=test.localhost \
APP_SECRET_CERTIFICATE_FILE=$HOME/tmp/private.pem \
go run cmd/app/main.go

❯ http test.localhost:8080
HTTP/1.1 200 OK
Content-Length: 80
Content-Type: application/json
Date: Fri, 04 Jul 2025 00:49:50 GMT

{
    "msg": "please check documentation for usage: https://github.com/octo-sts/app"
}

[cpanato]

thanks