RequestMetadata: Make tool_details repeated

[ColdrickSotK]

It is possible for more than one tool to be involved in the sending of a gRPC request, for example when a client-side proxy is used it is interesting to know the name and version of both the proxy and the original client tool.

To facilitate this in RequestMetadata, this PR updates the tool_details field to be repeated, allowing proxies and other related tools to also be recorded in the metadata sent with requests without modifying the original values.