instantaneous_ops_per_sec sometimes negative after 'CONFIG RESETSTAT'

[yoav-steinberg]

instantaneous_ops_per_sec sometimes negative after CONFIG RESETSTAT
tested on v2.8.6
To recreate, in one terminal window run this python code

import redis
r = redis.Redis()
import time
while True:
    r.ping()
    time.sleep(0.1)

and in another terminal window run:
redis-cli config resetstat; redis-cli info stats
You might need to repeat the commands above a few times to see it, but eventually you'll get some output like:

redis-cli config resetstat; redis-cli info stats
OK
# Stats
total_connections_received:1
total_commands_processed:1
instantaneous_ops_per_sec:-2
rejected_connections:0
sync_full:0
sync_partial_ok:0
sync_partial_err:0
expired_keys:0
evicted_keys:0
keyspace_hits:0
keyspace_misses:0
pubsub_channels:0
pubsub_patterns:0
latest_fork_usec:0

Notice the negative ops_per_sec above.
This is most likely because we reset server.stat_numcommands and forget to reset server.ops_sec_last_sample_ops leading to negative values in the ops per sec calculation.