Durations all show as zeroes in Flux logging in 1.8

__Steps to reproduce:__
1. Start an `influxd` server with `flux-enabled` and `flux-log-enabled` set to `true`
2. Execute a query
3. Observe a log message like this one:
```
Executed Flux query {"log_id": "0Ph0XEh0000", "service": "httpd", "compiler_type": "REPL", "response_size": 7251829, "query": "", "stat_total_duration": "0.000ms", "stat_compile_duration": "0.000ms", "stat_queue_duration": "0.000ms", "stat_plan_duration": "0.000ms", "stat_requeue_duration": "0.000ms", "stat_execute_duration": "0.000ms", "stat_max_allocated": 345856, "stat_concurrency": 0}
```

__Expected behavior:__
The durations in the stats should not be all zeroes, even if the query was really fast.

__Actual behavior:__
All the durations logged in the stats are zeroes.

__Environment info:__

* System info: `Darwin 19.6.0 x86_64`
* InfluxDB version: 1.8 (latest as of today)

__Config:__
```
flux-enabled = true
flux-log-enabled = true
```