feat(metrics): add Prometheus-compatible headers to metrics API

[pinylin]

Previously the metrics endpoint not included headers compatible with Promtheus.
Now adds 'Content-Type: text/plain' and other headers needed
for Prometheus scraping while maintaining backward

refer: # protocol-headers

[levkk]

Hey! Thanks for the PR and apologies for reviewing it late. Just small issue: could you return Result instead of using expect? I know the odds of this panicking are 0, but I'm just trying to make sure we don't have anything that can crash PgDog in production unexpectedly. Thanks!

One more administrative thing to discuss as well is a CLA. PgDog is an AGPL-licensed project and also a company. If you'd like to contribute, you are more than welcome to do so. If not, I completely understand!

Thanks!

[CLAassistant]

All committers have signed the CLA.