[configopaque] Implement fmt.GoStringer interface for String #16739

Job	Run time
govulncheck	29s
setup-environment	14s
lint	3m 12s
checks	1m 48s
test-coverage	2m 41s
cross-build-collector (darwin, amd64)	40s
unittest-matrix (~1.21.5)	2m 21s
cross-build-collector (darwin, arm64)	43s
cross-build-collector (linux, 386)	42s
unittest-matrix (~1.20.12)	1m 52s
cross-build-collector (linux, amd64)	36s
cross-build-collector (linux, arm64)	36s
cross-build-collector (linux, ppc64le)	38s
cross-build-collector (linux, arm, 7)	37s
cross-build-collector (linux, s390x)	40s
cross-build-collector (windows, 386)	37s
cross-build-collector (windows, amd64)	40s
unittest	1s
	19m 7s

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