fix(parsing): Group By clause in AST is now a List

[rahul-kumar-saini]

Overview

• Parser building the AST from a query creates a map() for the "group by" clause of a query
• This map is a kind of generator, which means if turned into a list somewhere down the line, the generator is drained and query.get_group_by() returns an empty map

Example

>>> from snuba.query.snql.parser import parse_snql_query_initial
>>> body = "MATCH (...) SELECT ... BY xyz ..."
>>> query = parse_snql_query_initial(body)
>>> print(query.get_groupby())
<map object at 0x110d21a60>
>>> print(list(query.get_groupby()))
[xyz]
>>> print(list(query.get_groupby()))
[]

• If at any time the map gets cast to a list, we lose the group by clause entirely in the AST

Changes

• Replaced map with list in the SnQL visitor when parsing group by clause

[volokluev]

can you add a test that would've broken before this change?

[codecov-commenter]

Codecov Report

Merging #2284 (d8e75e0) into master (6388143) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2284   +/-   ##
=======================================
  Coverage   93.06%   93.06%           
=======================================
  Files         553      553           
  Lines       25238    25244    +6     
=======================================
+ Hits        23487    23493    +6     
  Misses       1751     1751           

Impacted Files	Coverage Δ
snuba/query/snql/parser.py	96.12% <100.00%> (ø)
tests/query/test_query_ast.py	100.00% <100.00%> (ø)

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