Add supporting multi-target dynamic scraping

[iamhalje]

This PR introduces support for multi-target scraping via the /scrape?target=... endpoint, even when --mongodb.uri is configured globally. This allows dynamic discovery and monitoring of MongoDB instances without restarting the exporter, improving integration with Prometheus service discovery and multi-target configurations.

https://prometheus.io/docs/guides/multi-target-exporter/

scrape_configs:
  - job_name: 'mongodb_exporter_targets'
    metrics_path: /scrape
    static_configs:
      - targets:
          - mongodb://mongo-host1:27017
          - mongo-host2:27017
    relabel_configs:
      - source_labels: [__address__]
        target_label: __param_target
      - source_labels: [__param_target]
        target_label: instance
      - target_label: __address__
        replacement: <<MONGODB-EXPORTER-HOSTNAME>>:9216

Unlike the previous approach, where scraping was limited to the fixed list of instances specified by --mongodb.uri and when need added new host, need restart exported, this change enables flexible, on-demand querying of arbitrary MongoDB targets through the /scrape?target=... parameter.

Related issue: #878 — Multi target without need for specifying --mongodb.uri

Leave notes to the reviewers if you need to focus their attention on something specific:

This change affects core logic related to connection handling and target resolution. Existing setups using --mongodb.uri continue to work without changes. At the same time, users can now add dynamic targets seamlessly.
If you notice anything that could affect stability or how can do more good with code structure, caching or backward compatibility, especially in edge cases or other contributed rights, please let me know and we fixed this.

[it-percona-cla]

All committers have signed the CLA.

[iamhalje]

any feedback? can be produced