Add a second argument to callbacks for grpc streams with metadata

[cchamplin]

What?

Change updates the grpc stream callbacks to all receivers to be passed a meta data object that includes the timestamp of the original event. This change facilitates being able to accurately check the time when a message was received by a client stream.

Why?

During during load testing it may be important to know exactly when specific messages are received by the client to track various heuristics. At the moment this is difficult or impossible to achieve because the callbacks are only executed when the vu's tick, so there may be a floor of the vu internal for all measured timestamps on messages unless the time the message was received is captured by the k6 runtime and passed into the vu.

Checklist

• I have performed a self-review of my code.
• I have added tests for my changes.
• I have run linter locally (make lint) and all checks pass.
• I have run tests locally (make tests) and all tests pass.
• I have commented on my code, particularly in hard-to-understand areas.

Related PR(s)/Issue(s)

No issue exists yet

[codebien]

Hey @cchamplin,

During during load testing it may be important to know exactly when specific messages are received by the client to track various heuristics.

Can you expand more on your use case? What kind of logic do you want to apply based on each single stream's request? Did you try to implement the measurement directly in your JavaScript code?

The current proposed solution is very opinionated and specific for your case, it is worth exploring if we can do something a bit more generic.

Metadata is a very specific concept in the gRPC ecosystem, did you pick meta on purpose for the argument name?

[cchamplin]

@codebien We have a metric we are trying to capture that is essentially the around trip time it takes for certain events to arrive at a grpc client. At the moment we have not found a way to accurately capture this since we don't want to use a timestamp from the server and if we grab the data/time in the callback for the grpc stream message that may off by minIterationDuration or any amount of time the VU has slept. So we're looking for a way to add some metadata (general sense) around when exactly a message was received that will not be impacted by VU's sleeping. We're open to any solutions for this.

[codebien]

Hey, thanks for sharing. I thought a bit more about this and it makes sense. In the end, this is what we do with Response.timings on the HTTP module.

Let me sync internally regarding the specific API, I would prefer if we have something consistent between modules and gRPC functions (both unary and streaming).

[codebien]

Hey @cchamplin, the team synced on this pull request and we are fine with the current proposal. Please, can you rebase so we can merge it as-is?