chore: add /ready api for health checking

[shuiyisong]

I hereby agree to the terms of the GreptimeDB CLA.

Refer to a related PR or issue link (optional)

What's changed and what's your intention?

Add /ready HTTP endpoint for health checking. See https://grafana.com/docs/loki/latest/reference/loki-http-api/#readiness-probe.

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR requires documentation updates.

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[shuiyisong]

https://docs.victoriametrics.com/victorialogs/data-ingestion/vector/#loki
We can put /ready under /v1/loki prefix too, treating ip:4000/v1/loki as the whole endpoint, in case some other auto path is deduced and used.

Let's comply with Grafana's rules first.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.82%. Comparing base (c0f498b) to head (a6173ec).
Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5124      +/-   ##
==========================================
- Coverage   84.06%   83.82%   -0.25%     
==========================================
  Files        1167     1168       +1     
  Lines      217347   217378      +31     
==========================================
- Hits       182709   182212     -497     
- Misses      34638    35166     +528     

[fengjiachun]

LGTM