Floats in statsd percentiles

[Pitxyoki]

Required for all PRs:

• Signed CLA.
• Associated README.md updated.
• Has appropriate unit tests.

This addresses #5527. Note that this will be a breaking change for those using the plugin with the percentile array configured with integer numbers. It will now fail in those cases and only work with floats.

The already-present Unit Tests were only slightly changed to accept the correct type. No new tests were added.
A signed CLA should be on its way.

[glinton]

Can you make the pr against master?

[danielnelson]

For the backwards compatibility issue, we can define a number type. Place this in internal/internal.go:

type Number struct {
	Value float64
}

func (n *Number) UnmarshalTOML(b []byte) error {
	value, err := strconv.ParseFloat(string(b), 64)
	if err != nil {
		return err
	}

	n.Value = value
	return nil
}

Then use a []internal.Number as the type in the plugin.

[Pitxyoki]

Thank you for your feedback. This will continue in #5572.