Log details of downstream pipeline creation

What does this MR do and why?

• Related to https://gitlab.com/gitlab-org/gitlab/-/issues/285090 (implementation issue)
• Related to #358425 (closed) (feature flag issue)

In this MR we start logging when a downstream pipeline (multi-project or parent-child) is created. We log information about the hierarchy size and other metadata to help defining a proper hierarchy size limit that is not disruptive for GitLab SaaS.

For hierarchy size we intend the number of nodes present in a pipeline hierarchy starting from the top-level pipeline (the one that starts triggering all the downstream pipeline). For example, the following pipeline hierarchy will have size 6.

flowchart TD

PipelineA --> PipelineB1
PipelineA --> PipelineB2
PipelineB1 --> PipelineC1
PipelineB1 --> PipelineC2
PipelineB2 --> PipelineC3

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