Support structured logging in Cloud Run

[yuryu]

Cloud Run and Cloud Logging have structured logging support and it'd be nice to support it. Logrus supports custom json, so we just need to map fields.

json formatter: https://pkg.go.dev/github.com/sirupsen/logrus#JSONFormatter
special fields: https://cloud.google.com/logging/docs/agent/configuration#special-fields

Alternatively we can consider using stackdriver-gae-logrus plugin: https://github.com/andyfusniak/stackdriver-gae-logrus-plugin