Feature Request: emit a metric and log when VSchema contains unrecognized parameters

[maxenglander]

Feature Description

In some cases when the VSchema contains parameters which are not understood by VTGate, VTGate will happily accept the VSchema without emitting any metrics or logs indicating that the VSchema contains unknown fields.

To see evidence of this, apply a VSchema with an extra lookup vindex param:

{
  "sharded": true,
  "vindexes": {
    "my_idx": {
      "type": "lookup",
      "params": {
        "from": "my_col1",
        "table": "my_tbl.my_col_idx",
        "to": "keyspace_id",
        "unknown_param": "hello_world"
      },
      "owner": "messages"
    }
  }
}

As far as I can tell (could be wrong!) VTGate doesn't consider this unusual, although it's possible that may vary from one vindex type to another.

It would be great if VTGate could emit a metric and a logging statement when it encounters fields in the VSchema which it does not understand.

Could make this backwards compatible with a VTGate param like --vschema-strict-mode=disable|warn|error:

• Disable is current behavior.
• Warn emits logs and metrics.
• Error emits logs and metrics, and maybe does something else like panic or put VTGate into non-serving state.

Use Case(s)

Here's a use case where this kind of thing would be pretty useful:

1. A database VSchema is using a performance-impacting vindex param that VTGate understands, everything is good.
2. Database is rolled into a binary that for some reason doesn't understand that vindex param.
3. VTGate keeps processing queries, but performance takes a hit.

With the right metrics/logs, and operational team could catch this issue in a dev or staging database before it becomes a problem in prod.

[maxenglander]

Superceded by #13041