Add factories for SamplingResult

[anuraaga]

We can currently only implement a Sampler by calling SamplingResult.create. For results without attributes, we do return singletons from this method so it doesn't have any real overhead. But it's somewhat annoying to have to call create with a parameter and not so intuitive - every time I do it I still ctrl-click through the method to confirm it's not creating a new object. The alternative of creating a private static final in the Sampler seems redundant vs us providing some factories

SamplingResult.recordAndSample()
SamplingResult.recordOnly()
SamplingResult.drop()

[Rocksnake]

My understanding of your point of view is to divide the oncreate method into three to obtain the corresponding instance separately, and ensure that there is only one instance globally. To call oncreatewith a parameter is implemented through these three split methods.

Is my understanding correct? If it is, I am very glad to do it. Could you can assign it to me. If not, can you give me some guidance.

[anuraaga]

For backwards compatibility we can't remove the current create so would just add these new methods. We already have the singletons as returned by create so just have to return those from the factories. I think you are on the right page just reiterating to confirm.

Let's wait to hear what other maintainers think before making any change if we decide to go with it the help is appreciated :)

[jkwatson]

This seems ok to me. Making it clear that we're not creating a new object with these calls is a good idea.

We should be sure to javadoc the equivalence to the existing method, just so people don't get confused and think there is a difference in behavior.

[anuraaga]

Cool - @Rocksnake if you'd like to help that'd be great

[Rocksnake]

I tried the following according to my understanding, and obtained them separately for different factories：
SamplingResult.java

ImmutableSamplingResult.java

and I wrote the unit test in SamplingResultTest without errors.

Can you help me check if my understanding is correct? If it is correct, I will optimize the method of obtaining factories in the whole project.

[anuraaga]

@Rocksnake Basically looks good but I don't think we need the versions with Attributes, it's less common and create works well for it (it does actually create a new object).