Add billing and bandwidth metrics

[okdas]

Thanks, @DazWilkin, for an amazing exporter.

I believe there's an important feature missing here - to track of and monitor cost-related data.

1. Pending charges: https://www.vultr.com/api/#tag/billing/operation/pending-charges
2. Account balance and total pending charges: https://www.vultr.com/api/#tag/account/operation/get-account
3. Bandwidth (previous, current so far, projected): https://www.vultr.com/api/#tag/account/operation/get-account-bandwidth

If I'll get to this - happy to submit a PR, currently not a priority, though.

[DazWilkin]

Thanks for the feedback.

I agree it would be useful to expand the scope of the exporter.

I've implemented the functionality that I need; if I find time to add these metrics, I'll update the issue.

[DazWilkin]

1. Pending charges is not currently supported by the govultr SDK see Issue #355
2. Account balance and pending charges is implemented but I documented this incorrectly (Billing collector incorrectly uses Account methods #41)
3. Bandwidth is not currently supported by the govultr SDK see Issue #354

[DazWilkin]

See: vultr/govultr#356