Update proposal

* Simplified proposal - moved CT-per-sample to possible future extension instead of embedding within proposal
* Changed proposal to have a new `__temporality__` label instead of extending `__type__` - probably better to keep metric type concept distinct from metric temporality. This also aligns with how OTEL models it.
* Updated remote-write section - delta ingestion will actually be fully supported via remote write (since CT-per-sample is moved out of main proposal for now)
* Moved temporary `delta_rate()` and `delta_increase()` functions suggestion to discarded alternative - not sure this is actually necessary if we have feature flag for temporality-aware functions anyway
* Fleshed out implementation plan

Signed-off-by: Fiona Liao <fiona.liao@grafana.com>

Author: fionaliao