fix: pass headers as initial metadata to work around gRPC bug

[dero1to]

Fixes #544

Summary

• Work around gRPC Ruby bug (Missing interceptor changes to metadata for streaming requests(Ruby) grpc/grpc#22448) where interceptor metadata changes are silently dropped when return_op: true is used
• Return developer-token, login-customer-id, and linked-customer-id from ServiceLookup#headers as initial metadata instead of relying solely on MetadataInterceptor
• Fix minor indentation in MetadataInterceptor (unrelated cleanup)

Details

gapic-common calls gRPC with return_op: true, which causes merge_metadata_to_send to run before interceptors execute. This means MetadataInterceptor's metadata modifications are lost, resulting in DEVELOPER_TOKEN_PARAMETER_MISSING errors.

This change passes the headers via gax's metadata option (set at client initialization), which bypasses the interceptor ordering issue. MetadataInterceptor is kept as-is for backward compatibility and can be cleaned up once the upstream fix (grpc/grpc#42073) is released.

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