By setting Access-Control headers, the Prometheus metrics API can be
accessed by cross-origin javascript applications (e.g., an external
dashboard pulling Prometheus metrics).
This roughly comprises the following changes:
- index target pools by job instead of scrape interval
- make targets within a pool exchangable while preserving existing
health state for targets
- allow exchanging targets via HTTP API (PUT)
- show target lists in /status (experimental, for own debug use)