Snapshot summary metrics

[smaheshwar-pltr]

Feature Request

Iceberg supports optional snapshot metrics - https://iceberg.apache.org/spec/#metrics. It would be nice for DuckDB to populate basic snapshot metrics like total records, added records, total files, etc.

Further Context

Writers often populate these metrics in snapshot summaries, so that certain queries can be answered faster, and to provide useful metadata about the table's current and historical states.

DuckDB currently does not populate snapshot metrics on writes. So snapshots look like:

"snapshots" : [ {
    "sequence-number" : 1,
     // ...
    "summary" : {
      "operation" : "append"
    },
    // ...
  } ],

[smaheshwar-pltr]

Addressed by #529