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 94.28571% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.10%. Comparing base (6ea382c) to head (34e335c).

Files with missing lines	Patch %	Lines
parser.go	89.65%	2 Missing and 1 partial ⚠️
lexer.go	97.36%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #57      +/-   ##
==========================================
+ Coverage   89.05%   89.10%   +0.05%     
==========================================
  Files          11       11              
  Lines        3681     3727      +46     
==========================================
+ Hits         3278     3321      +43     
- Misses        306      308       +2     
- Partials       97       98       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.