fix upstream discovery args metadata field caused crash.

[tsuijie]

Please answer these questions before submitting a pull request, or your PR will get closed.

Why submit this pull request?

• Bugfix
• New feature provided
• Improve performance
• Backport patches

What changes will this PR take into?

Apisix and Apisix-seed support configuration of auto-discovery via Nacos with metadata field, which is an object instead of string, thus adding metadata field to discovery_args will cause apisix-dashboard to crash and unable to restart.
Error log message:
ERROR store/store.go:405 json unmarshal failed: json: cannot unmarshal object into Go struct field Upstream.discovery_args of type string

Related issues

None.

Checklist:

• Did you explain what problem does this PR solve? Or what new features have been added?
• Have you added corresponding test cases?
• [] Have you modified the corresponding document?
• Is this PR backward compatible? If it is not backward compatible, please discuss on the mailing list first

[codecov-commenter]

Codecov Report

Merging #2832 (21332ca) into master (e0290c8) will decrease coverage by 6.23%.
The diff coverage is n/a.

❗ Current head 21332ca differs from pull request most recent head 9ed6215. Consider uploading reports for the commit 9ed6215 to get more accurate results

@@            Coverage Diff             @@
##           master    #2832      +/-   ##
==========================================
- Coverage   77.49%   71.27%   -6.23%     
==========================================
  Files         139      198      +59     
  Lines        3746     7763    +4017     
  Branches      881      877       -4     
==========================================
+ Hits         2903     5533    +2630     
- Misses        843     1920    +1077     
- Partials        0      310     +310     

Flag	Coverage Δ
backend-e2e-test-ginkgo	65.12% <ø> (?)
backend-unit-test	49.95% <ø> (?)
frontend-e2e-test	70.35% <ø> (-7.15%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
api/internal/core/entity/entity.go	90.90% <ø> (ø)

... and 76 files with indirect coverage changes

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[khanhngobackend]

This is very beneficial for developing/testing custom discovery plugins. Please have a look guys.