[DOC] Create a user guide for metrics-generator

[knylander-grafana]

Metrics-generator has become one of Tempo's most popular features. We should provide documentaiton that walks people through:

• Basic set up
• Example use cases and their configurations

Be mindful of where people are using metrics-generator. For example, Grafana Cloud users won't have the configuration options that standalone Tempo and Grafana users will have.

Questions to answer:

• What are the use cases that we should document?
• What is the priorities of these use cases? Which one should we document first?

Use case list:

• Usage of dimension_mappings and how to reassign labels to span_metrics from existing span attributes

Related:

• Add a section into the metrics generator documentation detailing when you might want local metrics generation
• Include doc for native histograms
• https://github.com/grafana/tempo-squad/issues/272
• https://github.com/grafana/tempo-squad/issues/362
• https://github.com/grafana/tempo-squad/issues/214
• https://github.com/grafana/tempo-squad/issues/452
• https://github.com/grafana/tempo/issues/3376

### Tasks
- [ ] Define the use cases
- [ ] Prioritize which use cases to tackle first
- [ ] Document the use cases
- [ ] Publish content

[suddyv519]

I would love to see documentation and examples around the usage of dimension_mappings and how to reassign labels to span_metrics from existing span attributes

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