Add gRPC support for OTLP in addition to HTTP

[gayanper]

Please describe the feature request.
Add support for grpc transport protocol for OTLP implementation for metrics.

Rationale
Some of the tools such as Microsoft .NET Aspire Dashboard which can be used as a developer testing tool to run locally for verification of metrics and traces can only be used if all three traces, metrics and logs are transported using grpc protocol. So it would be nice to implement the support for transporting metrics using grpc in addition to http as well.

Additional context
https://learn.microsoft.com/en-us/dotnet/aspire/fundamentals/dashboard/overview

[jonatan-ivanov]

As far as I remember (according to the specs and the OTel maintainers), collector implementation must support all three transports (http/json, http/protobuf, grpc) so that they can talk to any clients. It's not the client (Micrometer) who should support all of them but the server. For example the OTel SDK only supports two out of three. I would try to open an issue to the Aspire Dashboard to support all the transport protocols.

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