feat(datastore): Add query profiling

[bhshkh]

Introducing ExplainOptions:

• When ExplainOptions is not used: Datastore plans and executes the query. Only returns the query results.
• When ExplainOptions.Analyze is false: Datastore plans but does not execute the query. Only returns planner stage information. Can be used to check that a query has the necessary indexes and understand which indexes are used.
• When ExplainOptions.Analyze is true: Datastore plans and executes the query. Returns query results, planner stage information and execution statistics.

samples at GoogleCloudPlatform/golang-samples#3742

[danieljbruce]

LGTM. I think @kolea2 mentioned that samples will be required as well so samples could be added in this PR or in a separate PR.

[bhshkh]

Samples at GoogleCloudPlatform/golang-samples#3742

[danieljbruce]

LGTM. I can do another review once the owl-bot PR comes in so that all the CI checks pass.

[bhshkh]

Do not merge until release freeze ends in mid April

[gkevinzheng]

Why is this not structured as a private struct explainOptions with a function ExplainOptions that returns an instance of explainOptions? With this structure the user would have to pass in stuff like ExplainOptions{Analyze: true} instead of ExplainOptions(true), right?

[bhshkh]

Right now, ExplainOptions contains only Analyze field. In the future, if more fields are added to ExplainOptions struct, user will no longer be able to use ExplainOptions(true).

[gkevinzheng]

Ahh I see, I was confused because this is a proto field, not an option in the Golang sense of the word.