Created Timestamp: Store CT in per series metadata storage

[bwplotka]

Proposal

Part of #14217

With the PRW 2.0 spec comes implementation and metadata per series logic. This unblocks using it to track CT without adding zero sample which has some drawbacks (e.g. potentially misleading synthetic sample that was NOT added by scrape and inability to use this in remote write).

Proposed here.

Acceptance Criteria

• CT is being tracked as part of internal metadata structure (per series).
• Behind feature flag (different than created-timestamp-zero-ingestion as those features might be considered orthogonal, or even colliding).