Document default registry being an anti-pattern.

[bwplotka]

Currently, we use it in all our examples: https://pkg.go.dev/github.com/prometheus/client_golang/prometheus

[beorn7]

I think it's fine for simple cases (like examples ;) but discouraged for more complex cases. If we make that clearer, we might be able to stick with simple examples.

[stale]

Hello 👋 Looks like there was no activity on this issue for the last 3 months. Do you mind updating us on the status? Is this still reproducible or needed? If yes, just comment on this PR or push a commit. Thanks! 🤗
If there will be no activity in the next 4 weeks, this issue will be closed (we can always reopen an issue if we need!).

[stale]

Closing for now as promised, let us know if you need this to be reopened! 🤗