[Metrics] Expose more parameters to `before_emit_metric`

[cleptric]

To empower users with more flexibility in the before_emit_metric hook, we should expose all parameters of a to-be-emitted metric.

We already expose key and tags; value and unit should also be added.

def before_emit(key, value, unit, tags):
	if key == "removed-metric":
		return False
	tags["extra"] = "foo"
	del tags["release"]
	return True

sentry_sdk.init(
    ...
    _experiments={
	"before_emit_metric": before_emit,
    }
)