config.Merge overwrites zero values from files if cli option is not provided

[moreirathomas]

Description

Given this config:

request:
  method: GET
  url: http://localhost:9999?delay=10ms
  queryParams:
    fib: 5
  timeout: 2s

runnerOptions:
  requests: 0 # run until globalTimeout
  concurrency: 99
  globalTimeout: 60s

config.Merge incorrectly defaults concurrency to config.Default()'s value. This is because zero-values are always overwritten even when they have meaning.

Suggestions

We could find a way to "flag" config options as set or unset before merging to apply the correct strategy and only overwrite only if not already explicitly set.