examples: fix plugin and metadata sample code

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (bb1f48a) 85.62120% compared to head (6b0e456) 85.50329%. Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #128 +/- ## =================================================== - Coverage 85.62120% 85.50329% -0.11791% =================================================== Files 188 188 Lines 16114 16114 =================================================== - Hits 13797 13778 -19 - Misses 1757 1772 +15 - Partials 560 564 +4 ``` | [Flag](https://app.codecov.io/gh/trpc-group/trpc-go/pull/128/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=trpc-group) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/trpc-group/trpc-go/pull/128/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=trpc-group) | `85.50329% <ø> (-0.11791%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=trpc-group#carryforward-flags-in-the-pull-request-comment) to find out more. [see 8 files with indirect coverage changes](https://app.codecov.io/gh/trpc-group/trpc-go/pull/128/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=trpc-group)

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