Add dumpinterval and json agent options

[debedb]

Description

Add two new agent options for the file-leak-detector Java agent:

• dumpinterval=N — Periodically dump all open file descriptors every N seconds using a daemon Timer thread. This is useful for monitoring file descriptor leaks in long-running services where you want continuous visibility without waiting for shutdown.

• json — Output dumps in single-line JSON format instead of plain text. Each dump is a single JSON line containing a timestamp, descriptor count, and an array of open descriptors with type, resource, thread, open time, and stack trace. This format is suitable for log aggregation systems (CloudWatch Logs, ELK, Splunk, etc.) and enables structured querying of leak data.

Example agent string:

-javaagent:file-leak-detector.jar=dumpinterval=5,json,dumpatshutdown

Sample JSON output:

{"timestamp":"2025-01-15T10:30:00.000Z","openDescriptors":3,"descriptors":[{"index":1,"type":"file","resource":"/tmp/leaky.tmp","thread":"main","openedAt":"...","stackTrace":["com.example.App.main(App.java:10)",...]}]}

The PR also includes a LeakyApp example under examples/ that demonstrates both options with a multi-threaded application that intentionally leaks file descriptors.

Testing done

• Built the agent with mvn -DskipTests package
• Ran the LeakyApp example with both plain text (run.sh) and JSON (run-json.sh) modes
• Verified periodic dumps appear at the configured interval
• Verified shutdown dump triggers on SIGTERM
• Verified JSON output is valid single-line JSON parseable by jq

Submitter checklist

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