* Add unknown guardian set index alert
* Add prometheus metrics to check if exists observations without txhash
* Add max sequence cache error prometheus metrics
### Summary
Before this PR, when validating a VAA, we were always using the latest guardian set index to verify the signatures.
With this PR, we're making the signature verification against `vaa.guardianSetIndex` instead.
Context: https://github.com/wormhole-foundation/wormhole-explorer/issues/104
### Testing plan
* Added unit test cases
* Also tested manually