Support gracefulStop in the externally-controlled executor

[agilob]

Brief summary

ERRO[0004] There were problems with the specified script configuration:
        - scenario xxx has configuration errors: gracefulStop is not supported by the externally controlled executor
        - scenario xxx has configuration errors: gracefulStop is not supported by the externally controlled executor
❯ ../k6 version
k6 v0.41.0 ((devel), go1.19.5, linux/amd64)

k6 version

v0.41.0

OS

Linux

Docker version and image (if applicable)

No response

Steps to reproduce the problem

Run k6:

    exec: exec,
    executor: 'externally-controlled',
    duration: performanceConstantLoadTime,
    // will be evenly spread across the `timeUnit` period.
    // rate: target,
    // timeUnit: timeUnit, // rate of requests spread over timeUnit => evenly spread 60 requests over 60 seconds => 1RPS
    // preAllocatedVUs: target,
    maxVUs: 1000,
    gracefulStop: '30s'

Expected behaviour

Documentation is incorrect or the functionality of external-executor missing

Actual behaviour

.

[na--]

IIRC, during the development of #1007, gracefulStop wasn't added to the externally-controlled executor because it was already quite complex and the whole project was taking ages. We apparently never got around to finishing it, so we are left with this restriction 😞

k6/lib/executor/externally_controlled.go

Lines 96 to 102 in 6c997ce

	func (mec ExternallyControlledConfig) Validate() []error {
	errors := append(mec.BaseConfig.Validate(), mec.ExternallyControlledConfigParams.Validate()...)
	if mec.GracefulStop.Valid {
	errors = append(errors, fmt.Errorf(
	"gracefulStop is not supported by the externally controlled executor",
	))
	}

Though, to be fair, I think this is the first time someone has asked for it 🤔 I'll change this issue to reflect that it is a missing functionality, so people can 👍 it and share their use cases.

And we need to fix our docs, for sure, so I added grafana/k6-docs#989 to not forget that.

[MattDodsonEnglish]

We just corrected the docs: grafana/k6-docs@744ebae

@agilob Thanks for pointing out that inaccuracy.