Add loki canary to helm chart

[trevorwhitney]

Is your feature request related to a problem? Please describe.
When I install Loki via the helm chart, how do I know it's working?

Describe the solution you'd like
Include the loki-canary helm chart with the loki helm chart.

Describe alternatives you've considered
Document installing both charts.

Additional context
Adding the canary allows us to add a test that uses it as well.

[dannykopping]

Hell yes!

[jeschkies]

@dannykopping would you have the alerts for the Canary?

[dannykopping]

This is what we use internally at Grafana Labs.

{
  name: 'canaries_alerts',
  rules: [
    {
      // End to end latency as seen by our canaries.
      alert: 'LokiCanaryLatency',
      expr: |||
        histogram_quantile(0.99, sum(rate(loki_canary_response_latency_seconds_bucket[5m])) by (le, namespace, cluster)) > 5
      |||,
      'for': '15m',
      labels: {
        severity: 'warning',
      },
      annotations: {
        message: |||
          {{ $labels.job }} is experiencing {{ printf "%.2f" $value }}s 99th percentile latency.
        |||,
      },
    },
  ],
},