feat: add more metadata related APIs to RequestContext

[justlorain]

What type of PR is this?

feat: A new feature

Check the PR title.

• This PR title match the format: <type>(optional scope): <description>
• The description of this PR title is user-oriented and clear enough for others to understand.

(Optional) Translate the PR title into Chinese.

补充常用的Get***函数

(Optional) More detail description for this PR(en: English/zh: Chinese).

en: Supplement usual Get function
zh(optional): 补充常用的Get***函数

Which issue(s) this PR fixes:

Fixes #368

[codecov]

Codecov Report

Base: 67.21% // Head: 67.32% // Increases project coverage by +0.11% 🎉

Coverage data is based on head (90c4a27) compared to base (efde5b6).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #375      +/-   ##
===========================================
+ Coverage    67.21%   67.32%   +0.11%     
===========================================
  Files           89       89              
  Lines         8573     8593      +20     
===========================================
+ Hits          5762     5785      +23     
+ Misses        2450     2447       -3     
  Partials       361      361              

Impacted Files	Coverage Δ
pkg/app/context.go	68.79% <100.00%> (+1.43%)	⬆️
pkg/common/compress/compress.go	83.80% <0.00%> (+2.85%)	⬆️

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