feat: support multi-instance tool metric source prefixing

[k-rister]

Summary

• When RICKSHAW_TOOL_ID is set in the environment, log_sample prefixes both the file_id and the metric source descriptor with the tool ID (e.g., sysstat-1sec::mpstat)
• Ensures CDM keeps metrics from different instances of the same tool separated for correct aggregation
• No-op when the env var is not set (backwards compatible with single-instance tools)
• Companion to the rickshaw multi-instance tool support PR which sets RICKSHAW_TOOL_ID during post-processing

Test plan

• Run with single-instance tools (no RICKSHAW_TOOL_ID set) — verify identical behavior
• Run with RICKSHAW_TOOL_ID=sysstat-1sec set — verify metric files and sources are prefixed with sysstat-1sec::
• Verify no double-prefixing when log_sample is called multiple times with the same desc hash

🤖 Generated with Claude Code

[k-rister]

The concept this is used to implement is under review.

[k-rister]

No longer needed — the existing tool-name field in rickshaw-index already picks up the tool ID from the data directory name, providing the breakout dimension without any toolbox changes.