chore: bump go.opentelemetry.io/otel/exporters/prometheus from 0.55.0 to 0.56.0 in /docs/user/integration/sample-app in the otel group

chore: bump go.opentelemetry.io/otel/exporters/prometheus from 0.55.0 to 0.56.0 in /docs/user/integration/sample-app in the otel group #79