cockpit: Implement PCP metrics channel in the Python bridge

The Python bridge still used the separate `cockpit-pcp` bridge for
metrics gathering. For us to remove the full C bridge implementation the
separate PCP bridge also has to be rewritten.

This rewrite is a more or less Python copy of the C implementation using
the Python PCP module. Even though the Python PCP module offers a
"higher level" fetchGroup API but preliminary testing has found this has
some issues with changing multi instance values and instances can not be
omitted with a fetch group (but this can be done in our own code)

Co-Authored-By: Allison Karlitskaya <allison.karlitskaya@redhat.com>
Co-Authored-By: Tomas Matus <tomatus777@gmail.com>

pyproject.toml

@@ -31,6 +31,10 @@ module = [
     "libvirt_qemu",
     "pika",
 
+    # run without pcp module types
+    "pcp",
+    "cpmapi",
+
     # run without gobject-introspection (used from cockpit-client for Gtk)
     "gi.*",
 
@@ -196,10 +200,13 @@ labels =
 sitepackages = True
 install_command = python3 -m pip install --no-index --no-build-isolation {opts} {packages}
 wheel_build_env = pkg
+setenv =
+  NO_QUNIT=1
 
 # All other environments (names like py311-lint, py36-pytest, etc) are isolated
 # from the system and get their packages from PyPI, according to the specific
 # test environment being requested.  We build the wheel in a common environment.
+# Skip PCP as the PyPi package provides no wheel, requires compilation and is outdated (5.0)
 [testenv]
 package = wheel
 wheel_build_env = venv-pkg
@@ -216,6 +223,7 @@ deps =
 allowlist_externals = test/static-code
 setenv =
   NO_QUNIT=1
+  PYTEST_ADDOPTS=-k 'not pcp'
 commands =
   pytest: python3 -m pytest -opythonpath= {posargs}
   lint: test/static-code --tap

src/cockpit/channels/__init__.py

@@ -20,6 +20,7 @@
 from .http import HttpChannel
 from .metrics import InternalMetricsChannel
 from .packages import PackagesChannel
+from .pcp import PcpMetricsChannel
 from .stream import SocketStreamChannel, SubprocessStreamChannel
 from .trivial import EchoChannel, NullChannel
 
@@ -35,6 +36,7 @@
     InternalMetricsChannel,
     NullChannel,
     PackagesChannel,
+    PcpMetricsChannel,
     SubprocessStreamChannel,
     SocketStreamChannel,
 ]