feat: add support for fill modifiers in binary operations

[andriibeee]

part of VictoriaMetrics/VictoriaMetrics#10598

Summary by cubic

Added support for fill modifiers (fill, fill_left, fill_right) in binary operations to set default values when one side is missing. This expands query syntax and improves control over join behavior.

• New Features
  • Parser recognizes fill, fill_left, and fill_right modifiers with numeric values, including negatives, inf, and NaN.
  • fill(x) sets both FillLeft and FillRight; fill_left(x) and fill_right(x) set sides independently.
  • Works alongside existing on()/group_* modifiers and is treated as a reserved identifier.
  • Output formatting includes the fill modifiers via appendModifiers.
  • Added tests for case-insensitive syntax, value parsing, and normalization.

^{Written for commit 14411c8. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 3 files

[codecov]

Codecov Report

❌ Patch coverage is 84.61538% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.57%. Comparing base (0dc0e2d) to head (1718c1f).
⚠️ Report is 13 commits behind head on master.

Files with missing lines	Patch %	Lines
parser.go	77.41%	7 Missing and 7 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #62      +/-   ##
==========================================
- Coverage   89.69%   89.57%   -0.13%     
==========================================
  Files          11       11              
  Lines        2969     3069     +100     
==========================================
+ Hits         2663     2749      +86     
- Misses        209      216       +7     
- Partials       97      104       +7     

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[Haleygo]

Thanks for the pull request, it looks good!

Just two things:

1. the fill modifiers should not work with and, or, unless (existing operators in PromQL), since the whole point of those operators is to filter series based on their presence or absence, nor with if, ifnot, default (new operators in MetricsQL).
   So in parseExpr(), the check

   metricsql/binary_op.go

   Lines 71 to 74 in a808ea4

   	func isBinaryOp(op string) bool {
   	op = strings.ToLower(op)
   	return binaryOps[op]
   	}

   is not sufficient to determine whether parseFillModifier is legitimate , a new sub-op check seems needed.

2. the optimizer check for binary expression must also consider fill modifier.

Update:
pushed above change in 02e980d,
1718c1f

[f41gh7]

LGTM