Polish custom metric names

Prepend "payment_" to all of our custom metric names to group
metrics from PaymentServer under one name space in Prometheus.

This results in the following metrics being exported:

- payment_processors_stripe_charge_attempts
- payment_processors_stripe_charge_successes
- payment_redemption_signatures_issued
- payment_redemption_voucher_redeemed

The non-custom http endpoints continue to be available under their
Prometheus style labelled names, e.g.
http_responses_total{instance="payments", path="v1/stripe/charge"}