Backward compatibility for fields without the "@" sign in the FT.AGGREGATE command

[BagritsevichStepan]

Starting from v1.21.4, we began printing a warning if a field in the FT.AGGREGATE command is specified without the @ sign because Redis rejects such requests. For example:

> FT.AGGREGATE index "*" GROUPBY 1 field REDUCE ...
search_family.cc:211] bad arguments: Field name 'field' should start with '@'. '@field' is expected

Now, we need to add a flag that allows the use of such fields (without @). If this flag is not set, the request should be rejected.

After the fix if the flag is set:

> FT.AGGREGATE index "*" GROUPBY 1 field REDUCE ...
1) 5  // The request is accepted
...

The problem is in the ParseFieldWithAtSign method, where LOGGING were added. Now depending on the flag we should reject or accept requests without '@' sign

[BagritsevichStepan]

Moved this from the #4530, since it is not related to the query engine

[romange]

I am sorry why are we removing backward compatibility?

[adiholden]

We defensively should not remove backward compatibility.
This was the original task and we just needed to separate it from the original issue.
Backward compatibility for search queries without '@':
search_family.cc:211] bad arguments: Field name 'foo' should start with '@'. '@foo' is expected
It worked for 1.21.4 and stopped for next versions. desired state - to optionally accept it with a runtime flag.

I will fix this. Thank you for noticing this Roman

[BagritsevichStepan]

Yes, I slightly misunderstood the task. I’ve updated the description to make the task clearer.