InfluxDB metrics provider

[gitirabassi]

Prometheus seems to be the only backed for metrics available at the moment. Would be possible to create an integration with InfluxDB or other metrics sources?

[gitirabassi]

How can I get started on this?

[stefanprodan]

@gitirabassi I'm getting close to merging #419 this allows for other metric providers to be implemented. In the API design doc #418 I've added an example of how InfluxDB could be used.

Tomorrow I'm going to fodem so I'll be mostly offline, please comment on the issue or PR, I look forward working with you to get InfluxDB support in Flagger.