g3proxy: add fluentd log in coverage CI

[codecov-commenter]

Codecov Report

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

Project coverage is 42.46%. Comparing base (1e76185) to head (634aec0).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #375 +/- ## ========================================== + Coverage 42.24% 42.46% +0.21% ========================================== Files 1091 1091 Lines 98631 98631 ========================================== + Hits 41670 41883 +213 + Misses 56961 56748 -213 ``` | [Flag](https://app.codecov.io/gh/bytedance/g3/pull/375/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=bytedance) | Coverage Δ | | |---|---|---| | [g3proxy](https://app.codecov.io/gh/bytedance/g3/pull/375/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=bytedance) | `42.46% <ø> (+0.21%)` | :arrow_up: | | [lib](https://app.codecov.io/gh/bytedance/g3/pull/375/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=bytedance) | `25.43% <ø> (ø)` | | 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=bytedance#carryforward-flags-in-the-pull-request-comment) to find out more.

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