keep histograms in envoy prometheus receiver (#6243) #13182

Job	Run time
setup-environment	1m 27s
gofmt	7m 36s
generate-metrics	5m 56s
coverage	12m 7s
cross-compile (binaries-darwin_amd64)	5m 23s
lint	11m 46s
cross-compile (binaries-darwin_arm64)	5m 20s
test	17m 2s
cross-compile (binaries-linux_amd64)	5m 29s
cross-compile (binaries-linux_arm64)	5m 18s
tidy	2m 30s
cross-compile (binaries-windows_amd64)	5m 23s
cross-compile (binaries-linux_ppc64le)	5m 9s
	1h 30m 26s

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