New client option DisableLogStderr to disable expensive log stderr from subprocess

[magodo]

I have a light weighted go-plugin client that talks to a terraform provider, mainly to import and read one or more terraform resources from the provider.

I expect this is anI/O bound task, instead of CPU intensive, as the client&provider will be waiting for the cloud platform API call in most of the time. However, the result turns out the CPU is also non-negligible.

The provider I'm talking to is the terraform-provider-azurerm, whose on initialization is a bit wordy that will print a bunch of logs. By doing some benchmark and profiling. I noticed that the go routine for logging stderr from the subprocess is consuming about 20% of the CPU time:

With this PR to disable the log stderr, especially the expensive parseJSON, we can get 20% performance improvement:

Ideally, we shall check whether the logger is a hclog.NullLogger and the config.Stderr is io.Discard, instead of introducing this new property. While I don't know if there is a way to do the first check (as the current default behaivor for null config.Logger is to set a logger to set a non-noop logger.

Also note that when this DisableLogStderr won't stop spawning the go routine at all since we still need to check whether the stderr hit EOF to tell whether the subprocess has ended.