[connector/servicegraph] Fix failed label does not work leads to servicegraph metrics error

[Frapschen]

Description: <Describe what has changed.> resolve: https://github.com/open-telemetry/opentelemetry-collector-contrib/issues/32018

Link to tracking Issue: fixes https://github.com/open-telemetry/opentelemetry-collector-contrib/issues/32018

Testing: <Describe what testing was performed and which tests were added.>

add a unit test to verify this bug fixed

Documentation: <Describe the documentation added