Feat: update Mackerel provider

[Takashi-kun]

Feature

I add the following features:

• support apiBase
  • https://mackerel.io/docs/entry/howto/install-agent
• add service_metadata and role_metadata
  • https://mackerel.io/api-docs/entry/metadata
• enable to import by service
  • I think it is convenient when an organization has a lot of services
• update document
  • add parameter service-name
  • add Getting started section

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[Takashi-kun]

@googlebot I signed it!