docs: 日本語での README の内容を追加し、インストール方法や使用例を明確化

[kazeburo]

PR Type

Documentation

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Description

• Localize README content into Japanese

• Add installation and usage guidance

• Document agent configuration and metrics

• Clarify latency metric calculation behavior

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Diagram Walkthrough

flowchart LR
  install["Installation"] 
  usage["Command usage"]
  agent["mackerel-agent configuration"]
  metrics["Published metrics"]
  install -- "enables" --> usage
  usage -- "configured in" --> agent
  agent -- "reports" --> metrics

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File Walkthrough

	Relevant files
Documentation	README.md Document Japanese setup, usage, and metric semantics          README.md Translate the plugin overview and operational behavior into Japanese. Add mkr installation instructions and a command usage example. Document mackerel-agent plugin configuration and restart steps. Define emitted metrics, units, and failed-request behavior. +61/-18

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

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PR Reviewer Guide 🔍

Here are some key observations to aid the review process:

⏱️ Estimated effort to review: 1 🔵⚪⚪⚪⚪

🧪 No relevant tests

🔒 No security concerns identified

⚡ Recommended focus areas for review Invalid agent config mackerel-agent.conf expects a string command value, not a command argument array. Using this example can prevent the agent from loading the plugin configuration; provide the complete command as a quoted string instead. command = ["/usr/local/bin/mackerel-plugin-httping", "--url", "https://example.com/", "--key-prefix", "example", "--count", "10", "--interval", "200"]

[github-actions]

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