Support Configurable TF_LOG environment variable >= Terraform CLI 0.15

[bflad]

Description

Post Terraform CLI 0.15, setting log levels outside TRACE is now considered reliable. Currently, terraform-exec hardcodes the equivalent of TF_LOG=TRACE when using (tfexec.Terraform).SetLogPath():

terraform-exec/tfexec/cmd.go

Lines 145 to 154 in 1bbcd0b

	// always override logging
	if tf.logPath == "" {
	// so logging can't pollute our stderr output
	env[logEnvVar] = ""
	env[logPathEnvVar] = ""
	} else {
	env[logPathEnvVar] = tf.logPath
	// Log levels other than TRACE are currently unreliable, the CLI recommends using TRACE only.
	env[logEnvVar] = "TRACE"
	}

Consumers, such as the terraform-plugin-sdk/v2 acceptance testing framework should be able to pass along a provider developer's desire for DEBUG or higher logs only from Terraform CLI.

Proposal

• Add tfexec.Terraform type log string field.
• Add (tfexec.Terraform).SetLog(string) error method, which sets the log field if Terraform CLI >= 0.15.
• Update (tfexec.Terraform).SetLogPath(string) to set the log field to TRACE, unless it is already set (this will ensure SetLog() or SetLogPath() can be called in any order and preserve the previous behavior)
• Update (tfexec.Terraform).buildEnv() logic to call env[logEnvVar] = tf.log instead of env[logEnvVar] = "TRACE"

References

• Related: Support TF_LOG_{CORE,PROVIDER} #101