Add a new function parseJSON to --template flag

[superbrothers]

parseJSON function parses string as JSON.

For example, if an application outputs the following log message:

{"level": "INFO", "message": "This is log message"}

We can use parseJSON function to format logs as follows:

$ stern . --template='{{.PodName}}/{{.ContainerName}} {{with $d := .Message | parseJSON}}[{{$d.level}}] {{$d.message}}{{end}}{{"\n"}}'
app-544bbcd5dc-fwgdj/application [INFO] This is log message

You can try this new function using the following steps:

cat <<EOL | kubectl apply -f-
apiVersion: apps/v1
kind: Deployment
metadata:
  name: app
spec:
  selector:
    matchLabels:
      app: app
  template:
    metadata:
      labels:
        app: app
    spec:
      containers:
      - image: busybox
        name: application
        command: ["/bin/sh"]
        args:
        - -c
        - |-
          while true; do
            echo '{"level": "INFO", "message": "This is log message"}'
            sleep 1
          done
EOL

stern . --template='{{.PodName}}/{{.ContainerName}} {{with $d := .Message | parseJSON}}[{{$d.level}}] {{$d.message}}{{end}}{{"\n"}}'

Closes #22

[rkmathi]

Looks great!
this is very convenient😺