Use LabKit tracing configuration parsing when it supports OpenTelemetry

LabKit is the common library used across GitLab for bootstrapping distributed tracing into a Golang application.

Currently, it does not support OpenTelemetry protocol. There is already an MR opened about that in LabKit, but it requires further discussions.

In this Gitaly MR, Gitaly contains all the parsing logic to parse the configuration.

This issue is about using LabKit's parsing logic in Gitaly once this MR. Currently, this is not possible because the parsing logic in LabKit does not support OpenTelemetry.

Once it does, let's reduce duplicate code and use LabKit's parsing logic directly.