feat(exporter): implement subprocess execution with active stall watchdog and TCP metrics (#26)

- Replace in-process iperf3 C binding with subprocess execution
- Add active I/O stall detection and connect-timeout
- Expose TCP retransmissions, mean RTT (ms), and exporter cycle timestamp
- Upgrade Dockerfile to Python 3.12 running as non-root user (nobody)
- Fix testTimeout/testDuration environment variable mapping in Helm chart
- Remove dead TrueCharts prometheus-operator chart dependency
- Add unit tests for exporter metric parsing and stall handling
- Switch tooling from devbox to mise

Co-authored-by: Malarbase <malarbase@gmail.com>

exporter/Dockerfile

@@ -1,34 +1,24 @@
 # Stage 1: Build stage with dependencies
-FROM python:3.9-slim as builder
+FROM python:3.12-slim AS builder
 
-# Declare TARGETARCH for use in this stage
-ARG TARGETARCH
 WORKDIR /app
 
-# Minimal dependencies for builder stage if any Python packages had C extensions.
-# Assuming requirements.txt does not need gcc or other build tools for now.
-# If pip install fails later, add necessary build tools (e.g., gcc, python3-dev) here.
-RUN apt-get update && \
-    # apt-get install -y --no-install-recommends gcc python3-dev # Example if needed
-    rm -rf /var/lib/apt/lists/*
-
 # Install Python dependencies
 COPY requirements.txt .
-RUN pip install --no-cache-dir -r requirements.txt
+RUN pip install --no-cache-dir --prefix=/install -r requirements.txt
 
 # Stage 2: Final runtime stage
-FROM python:3.9-slim
+FROM python:3.12-slim
 
 WORKDIR /app
 
-# Install iperf3 and its runtime dependency libsctp1 directly in the final stage.
-# This simplifies the Dockerfile by removing the need to copy iperf3 components from the builder.
+# Install iperf3
 RUN apt-get update && \
-    apt-get install -y --no-install-recommends iperf3 libsctp1 && \
+    apt-get install -y --no-install-recommends iperf3 && \
     rm -rf /var/lib/apt/lists/*
 
 # Copy installed Python packages from the builder stage
-COPY --from=builder /usr/local/lib/python3.9/site-packages /usr/local/lib/python3.9/site-packages
+COPY --from=builder /install /usr/local
 
 # Copy the exporter application code
 COPY exporter.py .
@@ -36,5 +26,9 @@ COPY exporter.py .
 # Expose the metrics port
 EXPOSE 9876
 
+# Run as non-root user (nobody)
+USER 65534:65534
+
 # Set the entrypoint
 CMD ["python", "exporter.py"]
+