perf: parse Loki labels in protobuf write path

[shuiyisong]

I hereby agree to the terms of the GreptimeDB CLA.

Refer to a related PR or issue link (optional)

#5287

What's changed and what's your intention?

Parse Loki labels directly in protobuf write path instead of converting and decoding using JSON5, roughly 10x better performance-wise while ensuring correctness.

loki_labels/json5       time:   [3.9928 µs 3.9939 µs 3.9950 µs]
Found 4 outliers among 100 measurements (4.00%)
  2 (2.00%) low mild
  2 (2.00%) high severe
loki_labels/hand_parse  time:   [392.19 ns 392.54 ns 392.93 ns]
Found 2 outliers among 100 measurements (2.00%)
  1 (1.00%) high mild
  1 (1.00%) high severe

PR Checklist

Please convert it to a draft if some of the following conditions are not met.

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR requires documentation updates.
• API changes are backward compatible.
• Schema or data changes are backward compatible.

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[codecov]

Codecov Report

Attention: Patch coverage is 93.81443% with 6 lines in your changes missing coverage. Please review.

Project coverage is 83.87%. Comparing base (0ee4133) to head (ce7f7dd).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5305      +/-   ##
==========================================
- Coverage   84.11%   83.87%   -0.24%     
==========================================
  Files        1202     1202              
  Lines      225933   226022      +89     
==========================================
- Hits       190033   189575     -458     
- Misses      35900    36447     +547     

[killme2008]

Geat work! A few suggestions applied.