Fix CreateMessageRequestParams.Metadata type to JsonObject #2199

Job	Run time
build (ubuntu-latest, Release)	6m 35s
build (macos-latest, Release)	7m 8s
build (ubuntu-latest, Debug)	6m 34s
build (macos-latest, Debug)	7m 4s
build (windows-latest, Release)	11m 29s
build (windows-latest, Debug)	12m 9s
publish-coverage / publish-coverage	1m 9s
	52m 8s

Provide feedback