Consolidate RPC metadata attributes

[lmolkova]

Fixes #2869

Rename

• rpc.grpc.request.metadata and rpc.connect_rpc.request.metadata to rpc.request.metadata
• rpc.grpc.response.metadata and rpc.connect_rpc.response.metadata to rpc.response.metadata

[Copilot]

Pull request overview

This PR consolidates protocol-specific RPC metadata attributes into common, generic RPC attributes. The change promotes reusability across different RPC protocols by renaming rpc.grpc.request.metadata, rpc.connect_rpc.request.metadata to rpc.request.metadata, and their corresponding response metadata attributes to rpc.response.metadata. This is a breaking change that simplifies the semantic conventions and reduces duplication.

Key changes:

• Created new generic rpc.request.metadata and rpc.response.metadata attributes in the main RPC registry
• Deprecated the four protocol-specific metadata attributes (gRPC and Connect RPC variants)
• Updated all span definitions and documentation to reference the new generic attributes

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
model/rpc/registry.yaml	Added new generic rpc.request.metadata and rpc.response.metadata attributes with proper type, stability, brief, note, and examples; removed the protocol-specific variants
model/rpc/spans.yaml	Updated attribute references in Connect RPC and gRPC span definitions (client and server) to use the new generic metadata attributes
model/rpc/deprecated/registry-deprecated.yaml	Added deprecation entries for all four protocol-specific metadata attributes with proper renamed_to references
docs/rpc/grpc.md	Updated documentation tables and footnotes to reference the new generic RPC metadata attributes instead of gRPC-specific ones
docs/rpc/connect-rpc.md	Updated documentation tables and footnotes to reference the new generic RPC metadata attributes; also fixed example value formatting inconsistency (string to array)
docs/registry/attributes/rpc.md	Updated attribute registry to show new generic metadata attributes and moved deprecated attributes to the deprecated section
.chloggen/rpc-metadata.yaml	Added changelog entry documenting this breaking change