Update user_events metrics exporter to reflect new schema #3205

Job	Run time
cargo-deny	42s
coverage	16m 21s
docs	11m 51s
lint	14s
msrv	4m 5s
non-default-examples (opentelemetry-otlp/examples/external-otlp-grpcio-async-std)	8m 43s
non-default-examples (opentelemetry-otlp/examples/basic-otlp)	1m 12s
test (stable)	38m 7s
test (beta)	27m 53s
	1h 49m 8s

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