fix(aggregateSender): close transport on reload

[linkdata]

Summary

• retain the aggregate sender HTTP transport so it can be closed explicitly
• close the previous idle connection pool when aggregate sender config reloads
• avoid leaking reusable HTTP connections across reloads

Tests

• go test ./pkg/runner ./...

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Reviewing files that changed from the base of the PR and between b615285 and 012ced5.

📒 Files selected for processing (2)

• pkg/runner/aggregate_sender.go
• pkg/runner/runner.go

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[linkdata]

Additional context from comment files

These notes were split from the local markdown comment files and attached here because they describe this PR's change.

HTTP Client Transport Resource Leak on Reload

• Bug: In setupHistogramSender(), when the aggregateSender was replaced (e.g., during config reload), the old aggregateSender's HTTP transport's connection pool was never closed. The transport holds idle connections that accumulate over time.
• Impact: Over multiple config reloads, idle HTTP connections accumulate and are not returned to the OS, exhausting per-process connection limits and file descriptors.
• Fix: (1) Added httpTransport field to aggregateSender struct to track the transport. (2) In setupHistogramSender(), call CloseIdleConnections() on the old transport before replacing it.
• Reasoning: HTTP transports maintain connection pools that must be explicitly cleaned up when the client is replaced, or connections will remain open until GC runs or the process exits.
• Tests: Verified by running full test suite with no regressions.