fix: Update instrumentation pg to support merge statements

[ericmustin]

Summary

In postgres15+, the MERGE statement was added

https://www.postgresql.org/docs/current/sql-commands.html
https://www.postgresql.org/docs/current/sql-merge.html

we don't support it currently in our hard coded list of pg sql command constants(which was contributed before MERGE was added, iiuc), which results in missing db.operation attributes and db.statement attributes that look like ; on pg spans for merge statements, which isn't very useful.

This PR adds MERGE to our constants list, and also cleans up some duplicate entries :)

Notes

This was surfaced in cncf slack, i think my tests should pass but letting the ci do the work.

Hope everyone is doing well.