Describe the bug
When metadata is "preseeded" (constant per request), ghz adds repeated grpc-accept-encoding=gzip headers (each request keeps appending to the same object so it's duplicated 1000s of times).

To Reproduce
ghz--metadata='{"my-header":"my-value"}' --call <request> -B <data> <url> --enable-compression

Expected behavior
grpc-accept-encoding=gzip is set a single time on the metadata.

Environment

• OS:
• ghz: v0.118.0

Additional context
I believe this https://github.com/bojand/ghz/blob/master/runner/worker.go#L99-L101 is the problematic line - when the MD object is shared accross all requests it infinitely appends to the same object.