add aof incr file size

[dudizimber]

fix #977

Add a new opt-in metric to monitor AOF incr file sizes.

[oliver006]

I think this is fundamentally broken as it assumes that the exporter has access to the appenddirname directory which is not necessarily true, putting it in the exporter doesn't seem right.
There are probably other ways for you to expose this metric (shell script and node exporter?).

[dudizimber]

The permission issue is a valid point, so the metric requires the flag -include-aof-file-size to be set for the exporter to run.

Although it's possible to expose the metric with node_exporter, bundling it with the rest of Redis's metrics makes more sense. Moreover, exposing it through node_exporter is a cumbersome process and requires installation and scraping, which is not always guaranteed. redis_exporter is the standard tool for exporting Redis's metrics, so it's safer to assume that it will be used to handle Redis-related metrics.

[oliver006]

The exporter currently works exclusively via the network connection that the exporter opens to the valkey/redis instance, this change would break that paradigm.
As I pointed out above, you can add this independently from the exporter to e.g. the node_exporter that you might be running on your valkey/redis node and gather the data like that.