contrib/google.golang.org/grpc: Whitelist metadata-tags

[bendiknesbo]

The interceptors in contrib/google.golang.org/grpc exposes two functions to add tags for the span based on the gRPC metadata: func WithMetadataTags() Option and func WithIgnoredMetadata(ms ...string) Option

This pattern only allows for a block-list methodology for converting Metadata to Tags, not an allow-list, or any other more sophisticated pattern.

I have three different suggestions:

1. Add another option func WithMetadataDecider(decider func(k string, v []string) bool) Option or similar, that allows the caller (creator of the interceptor) to decide which metadata should be converted to a tag.
2. Add another option func WithRecognizedMetadata(ms ...string) Option, that would take precedence over WithIgnoredMetadata, only adding the metadata present in the cfg.recognizedMetadata.
3. Another suggestion might be to create an option that would pass in the ctx context.Context, req interface{} and info *grpc.UnaryServerInfo, to allow for any custom tagging, and free selecting of the tag-key, etc.