pg_statements query size hardcoded to 2048

Regardless of the value of track_activity_query_size the pg_statements truncates query up to 2048 characters: https://github.com/percona/pmm/blob/main/agent/agents/postgres/pgstatstatements/utils.go#L77.

That value should be aligend with 

Also; it causes a flood of warnings like:

for larger queries. Note that ... part comes from the truncate module.