feat: Implement iperf3 exporter core logic and log level configuration (#13)

* feat: Implement iperf3 exporter core logic and log level configuration

This commit completes the core functionality of the iperf3 exporter and adds flexible log level configuration.

Key changes:
- Added command-line (`--log-level`) and environment variable (`LOG_LEVEL`) options to configure the logging level.
- Implemented the main test orchestration loop (`main_loop`) which:
    - Discovers iperf3 server pods via the Kubernetes API.
    - Periodically runs iperf3 tests (TCP/UDP) between the exporter pod and discovered server pods.
    - Avoids self-testing.
    - Uses configurable test intervals, server ports, and protocols.
    - Requires `SOURCE_NODE_NAME` to be set.
- Refined the `parse_and_publish_metrics` function to:
    - Accurately parse iperf3 results for bandwidth, jitter, packets, and lost packets.
    - Set `IPERF_TEST_SUCCESS` metric (0 for failure, 1 for success).
    - Zero out all relevant metrics for a given path upon test failure to prevent stale data.
    - Handle UDP-specific metrics correctly, zeroing them for TCP tests.
    - Improved robustness in accessing iperf3 result attributes.
- Updated the main execution block to initialize logging, start the Prometheus HTTP server, and invoke the main loop.
- Added comprehensive docstrings and inline comments throughout `exporter/exporter.py` for improved readability and maintainability.

These changes align the exporter's implementation with the details specified in the design document (docs/DESIGN.MD).

* feat: Update Helm chart and CI for exporter enhancements

This commit introduces updates to the Helm chart to support log level
configuration for the iperf3 exporter, and modifies the CI workflow
to improve image tagging for pull requests.

Helm Chart Changes (`charts/iperf3-monitor`):
- Added `exporter.logLevel` to `values.yaml` (default: "INFO") to allow
  you to set the exporter's log level.
- Updated `templates/exporter-deployment.yaml` to use the
  `exporter.logLevel` value to set the `LOG_LEVEL` environment
  variable in the exporter container.

CI Workflow Changes (`.github/workflows/ci.yaml`):
- Modified the Docker image build process to tag images built from
  pull requests with `pr-<PR_NUMBER>`.
- Ensured that these PR-specific images are pushed to the container
  registry.
- Preserved existing tagging mechanisms (e.g., SHA-based tags).

* fix: Add Docker login and permissions to CI workflow

This commit fixes the Docker image push failure in the CI workflow
by adding the necessary Docker login step and ensuring the correct
permissions are set for the GITHUB_TOKEN.

- Added a Docker login step using `docker/login-action@v3` to the
  `Build Docker Image` job in `.github/workflows/ci.yaml`. This
  authenticates to GHCR before attempting to push images.
- Added a `permissions` block to the `Build Docker Image` job, granting
  `packages: write` scope to the `GITHUB_TOKEN`. This is required
  to allow pushing packages to the GitHub Container Registry.

---------

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>

.github/workflows/ci.yaml

@@ -27,6 +27,9 @@ jobs:
   build:
     name: Build Docker Image
     runs-on: ubuntu-latest
+    permissions:
+      contents: read # Needed to checkout the repository
+      packages: write # Needed to push Docker images to GHCR
     steps:
       - name: Check out code
         uses: actions/checkout@v3
@@ -42,12 +45,28 @@ jobs:
         uses: docker/metadata-action@v4
         with:
           images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
+          tags: |
+            # Tag with the PR number if it's a pull request event
+            type=match,pattern=pull_request,value=pr-{{number}}
+            # Tag with the git SHA
+            type=sha,prefix=
+            # Tag with 'latest' if on the main branch (though this workflow only runs on PRs to main)
+            type=ref,event=branch,pattern=main,value=latest
+
+      - name: Log in to GitHub Container Registry
+        uses: docker/login-action@v3
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
 
       - name: Build Docker image
         uses: docker/build-push-action@v4
         with:
           context: ./exporter
-          push: false # Do not push on PRs
+          # Push the image if the event is a pull request.
+          # The workflow currently only triggers on pull_request events.
+          push: ${{ github.event_name == 'pull_request' }}
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}
           platforms: linux/amd64,linux/arm64