tracing: Make service_name configurable via GITLAB_TRACING

Igorrequested to merge
tracing-service-name into master

This is the golang counterpart to https://gitlab.com/gitlab-org/labkit-ruby/-/merge_requests/39.

It allows the service name to be overridden by providing a service_name parameter in the GITLAB_TRACING env var.

refs https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/11626

cc @mwasilewski-gitlab

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