Tracking: Send event each time we show nudge #_1

Following https://about.gitlab.com/handbook/product/feature-instrumentation/#taxonomy

The event would be:

se_category: 'projects:merge_requests:show'

se_action: (required) use default values

se_label: no_pipeline_noticed

se_property: admin, maintainer, developer, or owner

se_value: not used

Frequency displayed: once per session

Add_Pipeline_Nudge__1

/cc @dstull @jackie_fraser @pcalder

Edited by Gayle Doud