Copy the gRPC metadata before modifying it.

[tomwilkie]

Fixes #12

[Malayks]

I did not face this problem yet but any reason it can be only from client code? Server modifies the metadata as well.

[tomwilkie]

This happens when a gRPC instrumented server triggers two gRPC calls as the result of a single incoming RPC. So I don't think it will happen in the server.

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[tomwilkie]

I signed it!

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[Malayks]

I guess the two gRPC calls should be goroutines that share the same context?

[tomwilkie]

They are - see here: https://github.com/weaveworks/cortex/blob/master/distributor/distributor.go#L321

[bhs]

@tomwilkie thank you for the fix. LGTM.