Missing pipeline_stages definition causes unexpected behaviour

[dannykopping]

Describe the bug
When defining a scrape_configs entry, if the pipeline_stages key is omitted then a docker pipeline stage will be created by default.

The definition can be found here:

// DefaultScrapeConfig is the default Config.
var DefaultScrapeConfig = Config{
	PipelineStages: []interface{}{
		map[interface{}]interface{}{
			stages.StageTypeDocker: nil,
		},
	},
}

...

// UnmarshalYAML implements the yaml.Unmarshaler interface.
func (c *Config) UnmarshalYAML(unmarshal func(interface{}) error) error {

	*c = DefaultScrapeConfig

	type plain Config
	if err := unmarshal((*plain)(c)); err != nil {
		return err
	}

	if len(c.JobName) == 0 {
		return fmt.Errorf("job_name is empty")
	}

	return nil
}

To Reproduce
Steps to reproduce the behavior:

1. Started Loki (HEAD)
2. Started Promtail (HEAD) with config:

...
scrape_configs:
- job_name: kernel logs
  static_configs:
    - targets:
        - localhost
      labels:
        job: kernel_logs
        __path__: /var/log/kern.log

Promtail is now configured with the docker pipeline stage, and produces errors:

level=info ts=2021-02-28T21:17:33.936395393Z caller=tailer.go:125 component=tailer msg="tail routine: started" path=/var/log/kern.log
ts=2021-02-28T21:17:33.936462167Z caller=log.go:124 component=tailer level=info msg="Seeked /var/log/kern.log - &{Offset:0 Whence:0}"
level=debug ts=2021-02-28T21:17:33.936771177Z caller=json.go:130 component=file_pipeline component=stage type=json msg="failed to unmarshal log line" err="ReadMapCB: expect { or n, but found F, error found in #1 byte of ...|Feb 28 09:0|..., bigger context ...|Feb 28 09:03:30 grafrican kernel: [570410.390357] d|..."
level=debug ts=2021-02-28T21:17:33.936845072Z caller=json.go:130 component=file_pipeline component=stage type=json msg="failed to unmarshal log line" err="ReadMapCB: expect { or n, but found F, error found in #1 byte of ...|Feb 28 09:0|..., bigger context ...|Feb 28 09:03:30 grafrican kernel: [570410.390359] d|..."
level=debug ts=2021-02-28T21:17:33.93690619Z caller=timestamp.go:176 component=file_pipeline msg="extracted data did not contain a timestamp"
level=debug ts=2021-02-28T21:17:33.9368872Z caller=json.go:130 component=file_pipeline component=stage type=json msg="failed to unmarshal log line" err="ReadMapCB: expect { or n, but found F, error found in #1 byte of ...|Feb 28 09:0|..., bigger context ...|Feb 28 09:03:30 grafrican kernel: [570410.390803] d|..."

This is consistent with processing of a docker stage:

Expected behavior
Logs to be processed with no pipeline stages, i.e. the scrape_configs entry should behave as if it were provided pipeline_stages: [].

Environment:
Irrelevant.

Screenshots, Promtail config, or terminal output
See above