MVP: Lead time for changes DORA Metric

[brunocribeiro]

Goal: The OpenDORA plugin is also capable of showing the Lead time for changes DORA metric.

Value: as it's one of the 4 more important DORA metrics, it's a MUST-HAVE for the plugin

Deliverables:

• Backend to provide the Gitlab Deployment Datafields using the out-of-the-box Apache Devlake integration
• The REST API endpoint is extended to support the ltc_* metric type and retrieve data for the frontend plugin
• The Backstage FE plugin can plot a chart for the Lead time for changes metric

Assumptions:

• The end-user/operator will setup the necessary connections to GitLab and provide the necessary transformations (if the defaults don't suffice) for determining PR -> deployment relationships.

Tasks

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1. [BE] REST API endpoint metric type to expose Lead Time for Changes #76
   +0
   
2. [BE] ltc metric type added to openapi document and mocks #84
   +0
   
3. [FE] Lead time for changes chart #85
   +0
   
4. [FE] Label for Lead time for changes description #95
   enhancement +1
   
5. [BE] Add ltc type to benchmark endpoint #94
   enhancement +1