Revert "Exclude the metrics fetching session's data from pg_stat_activity (#1185)"

[maxenglander]

This reverts commit 198454c.

prometheus-community#1185 appears to have broken some things:

{"time":"2025-09-10T03:57:53.677992156Z","level":"ERROR","source":"collector.go:223","msg":"collector failed","name":"long_running_transactions","duration_seconds":0.010053746,"err":"sql: Scan error on column index 1, name "oldest_timestamp_seconds": converting NULL to float64 is unsupported"}
{"time":"2025-09-10T03:57:53.679318983Z","level":"ERROR","source":"collector.go:223","msg":"collector failed","name":"process_idle","duration_seconds":0.011370523,"err":"pq: syntax error at or near ";""}