Middleware and metric to track per-database HTTP response egress (#5611)

# Description of Changes

This commit adds a new Prometheus metric,
`spacetime_http_response_size_bytes_total`, which tracks bytes sent as
responses to HTTP requests related to the database. This includes the
`sql` and `call` routes, guest-defined HTTP handlers, logs, plus some
misc. management routes. Notably, the `subscribe` route, which initiates
a long-lived WebSocket connection, is not counted by the new metric, as
we already track its egress separately.

The new metric is tracked by an Axum middleware.

# API and ABI breaking changes

N/a

# Expected complexity level and risk

1 or 2? I don't have a huge amount of confidence any time I touch our
Axum stuff, but this is just metrics, and (at least currently) not
billed metrics.

# Testing

- [x] New unit tests which assert that the middleware functions
correctly when used with a mocked router.
- [ ] I don't know how to integration-test metrics.

Author: gefjon