Add InfluxDB exporter to Actuator

[mklimasz]

Since 5688 issue wasn't done for a while, I decided to make my first contribution since I'm working on such configuration anyway.

Configurable implementation of gauge writer to export metrics into InfluxDB.

Fixes gh-5688

[philwebb]

Thanks!

[philwebb]

I polished this a little here. I'm wondering if we should strip out the Builder. It seems a little unnecessary since you can just configure InfluxDB directly. It's also not really a builder since it modifies the existing InfluxDB instance and doesn't create a new one.

[mklimasz]

My goal was to make clean configuration wrapper to avoid working on pure influxdb-java library.
But I guess, Builder isn't mandatory and can be removed or InfluxDB instance can be created inside builder (in this case we need as mandatory arguments for influxdb in Builder constructor, like url etc.).
I find 2nd approach better, since we need more informations, like db name. Also there is good option to add tag to Point (for example hostname). It is good for grouping metrics from different hosts, while allowing them to write to the same database (and data stay unique for every writer).

[philwebb]

Unfortunately I need to de-scope some issues targeted for Spring Boot 1.5. I'd like to push this back since we want to review metrics in general.

[mklimasz]

When you finish review metrics and you will find consensus, you can ping me here and I can remake this in correct way.

[izeye]

This looks superseded by #9970 as this is for #5688 which has been already superseded.

[snicoll]

Indeed, thanks @izeye !