Extend the agent and operator feature metrics with a helper that
registers their collectors into a Prometheus registry and returns a
gatherer.

This keeps the dumping logic in promdump generic while letting the
feature metrics packages control exactly which collectors are exposed.

Signed-off-by: Alasdair McWilliam <alasdair.mcwilliam@isovalent.com>

Add shared hidden Cobra handlers for `metrics dump features` in the
agent and operator feature metrics packages.

This keeps the dump implementation close to the agent and operator
feature metric gatherer construction, while leaving binary-specific
CLI registration to follow-up commits.

Signed-off-by: Alasdair McWilliam <alasdair.mcwilliam@isovalent.com>

Wire up the hidden dump handler beneath the existing `metrics`
command so feature metric generation lives next to `metrics list`.
This avoids introducing another top-level command on the operator
while keeping the operator-specific CLI wiring isolated to its own
commit.

Also move the sub-tree command registration from metrics_list.go
into metrics.go so the latter remains focused on the `list`
subcommand.

Signed-off-by: Alasdair McWilliam <alasdair.mcwilliam@isovalent.com>

Add new metrics command to the agent, which is wired up to expose the
hidden `metrics dump features` command for documentation regeneration.
This has been done to maintain command uniformity with the operator,
as per the previous commit.

Signed-off-by: Alasdair McWilliam <alasdair.mcwilliam@isovalent.com>

Add documentation make targets and helper scripts to regenerate and
check the feature metric reference files for the agent and operator.

The update script invokes the hidden `metrics dump features` command
and feeds the generated Prometheus output into the Helm feature
documentation generator.

Note: this has required adding tools/feature-helm-generator to the
top-level make target, so the generator is available to the update
helper script.

Signed-off-by: Alasdair McWilliam <alasdair.mcwilliam@isovalent.com>