Add a flag to disable exporter metrics

[isker]

The name of the flag and how it's implemented are cribbed from some other projects like node_exporter and systemd_exporter, though our situation is simpler than theirs.

Closes #8.
Closes #71 (by way of obsoleting it).

I tested this locally by ensuring that this includes version and exporter metrics (the ECS metrics of course aren't produced):

ECS_CONTAINER_METADATA_URI_V4=foobar go run main.go

While this only includes version metrics:

ECS_CONTAINER_METADATA_URI_V4=foobar go run main.go --web.disable-exporter-metrics

[SuperQ]

I typically prefer positive flags, but since this is the established pattern we'll stick to it for now.

[SuperQ]

This exporter acts more as a sidecar if I understand it correctly. I wonder if we should disable this by default, or perhaps tune the defaults to expose fewer metrics.

[isker]

It is a sidecar on every ECS task (analogous to k8s pod), as it exports metrics from an API only privately available within the task's network.

That seems not unreasonable to me.