feat: add mechanism for prefixing queries with a comment string

[kylemcc]

This change proposes a new Comment(string) method and ContextWithComment(context.Context, string) function for prepending a comment to a SQL query.

Here's simple example:

// using the query API
err := db.NewSelect().
    Model(&products).
    Where("description like ?", d).
    Comment("customer:"+customerID).
    Scan(ctx)

// using a context
qctx := bun.ContextWithComment(ctx, "customer:"+customerID)
err := db.NewSelect().
    Model(&products).
    Where("description LIKE ?", d).
    Scan(qctx)

This would produce a query that looks like:

/*customer:01J1P97P3AP7V287B57P7055CZ*/ SELECT * FROM products WHERE description LIKE '%red%'

This may warrant a bit of discussion first, but I wanted to include the code with the proposal. I have had this code running in production since April with no problems. It appears that others may be interested in this as well: #998

My immediate use case for this is to add a comment to a query with tracing/debug information that will show up in the process list/pg_stat_activity, but there are certainly other uses for this functionality.

[vmihailenco]

Could you please rework this to accept ctx context.Context and then extract the comment from the context in this function?

Then we can remove commentFromContext.

[kylemcc]

Thanks for the review @vmihailenco! Updated as you requested and rebased on top of master.

[vmihailenco]

@kylemcc This looks useful, but I have a minor refactoring request. Please take a look when you have time.

[vmihailenco]

Can you please change this so it does not modify the baseQuery? So the following is possible:

q := NewSelectQuery()
q.AppendQuery(ctxWithComment1) // one comment
q.AppendQuery(ctxWithComment2) // another comment

Also, one might think that both comments must be appended: one from the query and one from the context. I will leave it to you to decide what to do in such case.

[vmihailenco]

If multiple comments are allowed, then we can also consider supporting this: NewSelect().Comment("comment1").Comment("comment2").

[kylemcc]

Can you please change this so it does not modify the baseQuery? So the following is possible:

q := NewSelectQuery()
q.AppendQuery(ctxWithComment1) // one comment
q.AppendQuery(ctxWithComment2) // another comment

Also, one might think that both comments must be appended: one from the query and one from the context. I will leave it to you to decide what to do in such case.

Ah, yes, good catch. Didn't think about the AppendQuery use case.

If multiple comments are allowed, then we can also consider supporting this: NewSelect().Comment("comment1").Comment("comment2").

This seems reasonable, too. Let me experiment with the formatting a bit to see what looks best.

[vmihailenco]

@kylemcc any chance you can finish this?