[chore] Use mdatagen for configopaque and configtelemetry (#15696)

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#### Description
Completes the mdatagen migration for the `config/` packages.
`configopaque` and `configtelemetry` were the only two left using a
hand-written `package_test.go` after #15639.

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#### Link to tracking issue
No dedicated tracking issue 
this follows the same pattern established in #15632 and #15639. Happy to
open a tracking issue first if maintainers would prefer that.

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#### Testing
`go test -count=1 ./...` passes for both `config/configopaque` and
`config/configtelemetry`.


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#### Authorship

- [x] I, a human, wrote this pull request description myself.

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Author: inshining

config/configopaque/doc.go

@@ -19,3 +19,5 @@
 // or become widely used in the Go ecosystem, these will eventually be implemented
 // by configopaque.String as well. This is not considered a breaking change.
 package configopaque // import "go.opentelemetry.io/collector/config/configopaque"
+
+//go:generate mdatagen metadata.yaml

config/configopaque/package_test.go …g/configopaque/generated_package_test.goconfig/configopaque/package_test.go renamed to config/configopaque/generated_package_test.go config/configopaque/package_test.go renamed to config/configopaque/generated_package_test.go

@@ -40,3 +40,5 @@
 // The signals associated with this level may exhibit high cardinality, high data volume, or high
 // computational cost.
 package configtelemetry // import "go.opentelemetry.io/collector/config/configtelemetry"
+
+//go:generate mdatagen metadata.yaml