feat:length

[baerwang]

What this PR does:

Which issue(s) this PR fixes:

Fixes #502

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[codecov-commenter]

Codecov Report

Base: 35.71% // Head: 36.06% // Increases project coverage by +0.34% 🎉

Coverage data is based on head (6270d46) compared to base (b5e3e36).
Patch coverage: 82.35% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #508      +/-   ##
==========================================
+ Coverage   35.71%   36.06%   +0.34%     
==========================================
  Files         157      158       +1     
  Lines       21575    21593      +18     
==========================================
+ Hits         7706     7788      +82     
+ Misses      13055    12959      -96     
- Partials      814      846      +32     

Impacted Files	Coverage Δ
pkg/runtime/function2/length.go	82.35% <82.35%> (ø)
pkg/dataset/priority_queue.go	64.35% <0.00%> (-1.00%)	⬇️
pkg/boot/discovery.go	28.33% <0.00%> (+0.11%)	⬆️
pkg/schema/loader.go	6.74% <0.00%> (+1.12%)	⬆️
pkg/boot/boot.go	51.87% <0.00%> (+6.87%)	⬆️
pkg/boot/watcher.go	7.80% <0.00%> (+7.32%)	⬆️
pkg/boot/discovery_watch.go	24.74% <0.00%> (+24.74%)	⬆️

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[jjeffcaii]

PS: add test case for Chinese characters will better, for example: LENGTH('hello世界').