Calculate compute_backpressure_throttling_seconds correctly

The original value that we get is measured in microseconds. It comes
from a calculation using Postgres' GetCurrentTimestamp(), whihc is
implemented in terms of gettimeofday(2).

Signed-off-by: Tristan Partin <tristan@neon.tech>

compute/etc/sql_exporter/compute_backpressure_throttling_seconds.sql

@@ -1 +1 @@
-SELECT neon.backpressure_throttling_time()::float8 / 1000 AS throttled;
+SELECT (neon.backpressure_throttling_time()::float8 / 1000000) AS throttled;