Add load balancing Prometheus metrics in server_metrics middleware

What does this MR do?

Introduces new prometheus counter metrics in Sidekiq ServerMetrics Middleware:

• sidekiq_load_balancing_count: - Number of sidekiq jobs using load balancing with data consistency set to :sticky or :delayed

If a worker is configured with data_consistency set to :sticky or :delayed, metrics will increment if load balancing is enabled for Sidekiq, with combination of labels:

• data_consistency =delayed/sticky
• database_chosen = primary/retry/replica

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

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Related to #326225 (closed)