* docs: add API reference for all metric types

Adds constructor parameter tables, method documentation, and runnable
real-world examples for Counter, Gauge, Histogram, Summary, Info, and
Enum. The _index.md quick-pick table now covers all six types. Also
fixes labels.md which was missing remove(), remove_by_labels(), and
clear() -- the metric pages were already linking to it for those methods.

Closes #1021

Signed-off-by: k1chik <[email protected]>

* docs: address review feedback on counter example and table header

Revert quick-start counter example to use 'my_failures' (no _total suffix)
since the library appends it automatically. Rename 'Value goes' column to
'Behavior' in the metric type overview table for clarity across all six types.

Signed-off-by: k1chik <[email protected]>

* docs: rename table column to 'Update model'

Signed-off-by: k1chik <[email protected]>

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Signed-off-by: k1chik <[email protected]>