fix: [Geneva Uploader] Expose tenant identity as Geneva queryable fields

[lalitb]

Changes:

• Ensure Tenant, Role, and RoleInstance remain serialized as Bond schema fields so they appear as queryable columns in CentralBond, aligning with the C# ingestion pipeline.
• Precompute the CentralBlob metadata string in MetadataFields::new to avoid repeated formatting while keeping legacy metadata headers and source identity untouched.

With this change, the tenant information is send as:

1. Metadata string in CentralBlob → "tenant=XXX/role=YYY/roleinstance=ZZZ". <EXISTING>
2. sourceIdentity URL parameter → "Tenant=XXX/Role=YYY/RoleInstance=ZZZ" <EXISTING>
3. Tenant/Role/RoleInstance as Bond schema fields → making them queryable! <ADDED IN THIS PR>

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

❌ Patch coverage is 88.09524% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 54.0%. Comparing base (bf66cdd) to head (2a553e8).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...metry-exporter-geneva/geneva-uploader/src/bench.rs	0.0%	14 Missing ⚠️
...etry-exporter-geneva/geneva-uploader/src/client.rs	92.8%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #488     +/-   ##
=======================================
+ Coverage   53.8%   54.0%   +0.2%     
=======================================
  Files         71      71             
  Lines      11702   11782     +80     
=======================================
+ Hits        6300    6370     +70     
- Misses      5402    5412     +10     

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