Add example that uses kopf + kr8s

[jacobtomlinson]

Closes #235

Added an example that shows how to write an operator controller with kopf and kr8s together.

# controller.py
import re

import kopf
import kr8s
from kr8s.objects import Pod


@kopf.on.resume("pods")
@kopf.on.create("pods")
def add_os_labels(body, logger, **kwargs):
    pod = Pod(body)

    # Pod was deleted while trying to add OS labels, give up
    if not pod.exists():
        return

    # Pod already has OS labels, skip
    if "os-release/id" in pod.labels:
        return

    # Pod is not ready yet, retry in a bit
    if not pod.ready():
        raise kopf.TemporaryError(f"Pod {pod.name} is not ready yet", delay=10)

    # Get OS info
    try:
        output = pod.exec(["cat", "/etc/os-release"])
        os_info = output.stdout.decode()
    except kr8s.ExecError:
        logger.error(
            f"Failed to exec in pod {pod.name}, "
            "either cat is not included in the image or /etc/os-release is missing."
        )
        pod.label({"os-release/id": "unknown"})
        return

    # Clean OS labels
    labels = {}
    for label in os_info.splitlines():
        key, value = label.split("=")
        key = f"os-release/{key.lower().replace('_', '-')}"
        # Kubernetes only accepts label values with alphanumeric characters,
        # dots, dashes and underscores and a maximum length of 63 characters.
        value = re.sub("[^0-9a-zA-Z_.-]+", "", value)[:63]
        labels[key] = value

    # Apply OS labels
    pod.label(labels)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (985a2a3) 94.55% compared to head (cf2b950) 94.55%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #243   +/-   ##
=======================================
  Coverage   94.55%   94.55%           
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  Files          27       27           
  Lines        2701     2701           
=======================================
  Hits         2554     2554           
  Misses        147      147           

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