Add explain support for methods like last, pluck and count

[p8]

Motivation / Background

explain can be called on a relation to explain how the database would execute a query.
Currently explain doesn't work for queries using last, pluck or count, as these return the actual result instead of a relation. This makes it difficult to optimize these queries.

By letting explain return a proxy instead, we can support these methods.

Detail

User.all.explain.count
# => "EXPLAIN SELECT COUNT(*) FROM `users`"

User.all.explain.maximum(:id)
# => "EXPLAIN SELECT MAX(`users`.`id`) FROM `users`"

This breaks the existing behaviour in that it requires calling inspect after explain.

User.all.explain.inspect
# => "EXPLAIN SELECT `users`.* FROM `users`"

However, as explain is mostly used from the commandline, this won't be a problem as inspect is called automatically in IRB.

Additional information

This is a variation of #50424 which used arguments instead of a proxy and method calls.

Another variation could be passing a block instead. This would not break the current behaviour, but it's a bit more verbose:

User.all.explain do |relation|
   relation.count
end

With a symbol-to-proc it becomes a bit terser:

User.all.explain(&:count)

Unlike the proxy solution, this notation could also be applied to to_sql:

User.all.to_sql(&:count)

Or we could use method name variations, but that would break with the arguments that can be passed to explain, like explain(:analyze, :buffers):

User.all.explain_count
User.all.explain_maximum(:id)

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.

[rafaelfranca]

if we only support those methods why use method missing?

[p8]

I've updated the implementation to use regular methods. This also makes sure the method signatures match.

[fxn]

Is the first line in the CHANGELOG complete?

[p8]

@fxn Would you want to see all methods?
I think a previous version contained for methods like.

[fxn]

@p8 The current sentence is

Add explain support for last, pluck and count

As a reader, I'd interpret the support is for exactly those three.

Instead of "like", I believe it would be more informative to list them all. You read the CHANGELOG, and you know what is new without having to think more. See what I mean?

[p8]

@fxn I've created a PR to update the CHANGELOG: #50599