_timestamp index attribute needs to be unique for original published data

[durack1]

We currently only have a single _timestamp = 2023-05-12T14:48:11.983Z attribute in the ESGF index. This needs to be unique for every dataset, such that when a new replica (the same dataset, bit for bit) is republished, the original timestamp is preserved. A new e.g. _timestampReplica1 = 2023-12-12T19:48:11.983Z is generated allowing for this original version/timestamp to be used persistently, e.g. in the data citation/license info etc.

Example from https://esgf-node.llnl.gov/search/input4mips/?institution_id=PCMDI&source_id=PCMDI-AMIP-1-1-9

@MartinaSt @wolfiex @SebastienDenvil @sashakames ping

[MartinaSt]

Thoughts on clean-up:

1. ESGF index
2. Metagrid search for version implementation
3. data citation instructions

[sashakames]

Just catching up on this. the issue title might be better rewritten to express the concern. _timestamp is always unique because it reflects the index-assigned timstamp at a sub-second granularity. What we want in the publisher is to write the original creation time of the record to the replica record? IIRC from our chat at AGU?
eg. original_timestamp