: character in dynamic variables breaks the rendering

[ChrsMark]

It seems that having : character as part of dynamic variables' default value breaks the whole rendering.

A configuration like the following are not able to get rendered:

inputs:
  - name: templates.d/redis/0.3.6
    type: redis/metrics
    data_stream.namespace: default
    use_output: default
    streams:
      - data_stream:
          dataset: redis.info
          type: metrics
        metricsets:
         - info
        hosts:
          - "${kubernetes.hints.redis.info.host|'127.0.0.1:6379'}"

Running ./elastic-agent -e -c ./elastic-agent.yml inspect output -o default I get the following:

{"log.level":"error","@timestamp":"2022-06-27T15:31:21.461+0300","log.origin":{"file.name":"emitter/controller.go","file.line":123},"message":"Failed to render configuration with latest context from composable controller: starting ${ is missing ending }","ecs.version":"1.6.0"}

This is important in order to define default values for hosts settings.

[ChrsMark]

Note that the specific issue is important for #662. Since this might be probably an easy/small "fixup" issue, @jlind23 do you think it could be prioritised for 8.4?

[jlind23]

I'll let @pierrehilbert take a look at it.
If it is small enough @michel-laterman may be able to take a stab at it.

@pierrehilbert your call.