support variables, that are passed instead of numbers

[AndrewChubatiuk]

• support variables, that are passed instead of numbers
• add ParseWithVars, that preserves variables in expressions
• add label_values function support to parse grafana queries

using library to properly modify metricsql expressions imported rules and dashboards here. while parsing metricsql expression in grafana dashboards it fails when:

• unquoted variable is used as a function argument: histogram_quantile($quantile, sum(rate(http_request_duration_seconds_bucket[5m])) by (le))
• parsing expression, which contains grafana specific label_values function

[codecov]

Codecov Report

❌ Patch coverage is 94.91525% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.81%. Comparing base (a808ea4) to head (01c77e4).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
parser.go	91.30%	1 Missing and 1 partial ⚠️
lexer.go	96.96%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #57      +/-   ##
==========================================
+ Coverage   89.73%   89.81%   +0.07%     
==========================================
  Files          11       11              
  Lines        2981     3023      +42     
==========================================
+ Hits         2675     2715      +40     
- Misses        209      210       +1     
- Partials       97       98       +1     

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