OTLP metadata must be set as tuple

[codeboten]

Describe your environment
The current implementation of the OTLP exporter uses a string of key-value pairs when setting the value in the grpc connection, this is incorrect according to what the grpc glossary expects metadata to be a sequence of tuple
https://grpc.github.io/grpc/python/glossary.html#glossary:

2-tuple where the first entry is the key and the second is the value