Fix loss of precision in pg_stat_us_to_ms()

Fix loss of precision in pg_stat_us_to_ms()

Multiplying by the constant 0.001 can produce trailing-digit noise in
displayed values (for example 0.009000000000000001 instead of 0.009,
with default extra_float_digits) because 0.001 cannot be represented
exactly in binary floating point. Use division by 1000.0 instead,
matching code elsewhere in the tree.

Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Discussion: https://postgr.es/m/akIYkMK4bHe9qX/N@bdtpg