Add support for creating metrics from HTTP probe output

[manugarg]

I had opened ticket a long time back at the older location:
google/cloudprober#297

Recently while running a probe, I realized it can be quite a useful thing to be able to build metrics from HTTP probe's output.

[manugarg]

From the discussion on #508, I realized that we can probably go a step further and use HTTP headers for custom metrics as well. Something like:

output_metric_options {
  header_to_metrics {
    header: LastModified (LastModified | ContentLength)
    metric: last-modified
  }
  header_to_metrics {
    header: ContentLength
    metric: content-length
  }
  ...
}

[manugarg]

We can go a step further and provide an option generate metrics from JSON when output is JSON.