* 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 prometheus#1021

Signed-off-by: k1chik <107162115+k1chik@users.noreply.github.com>

* 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 <107162115+k1chik@users.noreply.github.com>

* docs: rename table column to 'Update model'

Signed-off-by: k1chik <107162115+k1chik@users.noreply.github.com>

---------

Signed-off-by: k1chik <107162115+k1chik@users.noreply.github.com>

Signed-off-by: Chris Marchbanks <csmarchbanks@gmail.com>

…rometheus#1162)

Closes prometheus#1161

Adds parameter tables and formal API reference sections to three pages
that previously had examples but no parameter documentation.

pushgateway.md: documents push_to_gateway, pushadd_to_gateway,
delete_from_gateway, instance_ip_grouping_key, and all four built-in
handlers (default, basic_auth, tls_auth, passthrough_redirect).

textfile.md: documents write_to_textfile with all four parameters
including the previously undocumented escaping and tmpdir, plus atomic
write semantics and error behavior.

multiprocess/_index.md: documents MultiProcessCollector constructor and
mark_process_dead with parameter tables including the previously
undocumented path parameter on both.

Signed-off-by: k1chik <107162115+k1chik@users.noreply.github.com>

Assigning the .time() context manager (with ... as t) now yields a Timer
whose .duration attribute holds the observed value in seconds after the
block exits. This lets callers reuse the measurement (logging, further
metrics) without calling default_timer() a second time.

Signed-off-by: Lukáš Vokráčko <lukas@vokracko.cz>

…asses (prometheus#1169)

Closes prometheus#1163

collector/custom.md: Collector protocol section (collect/describe), value vs
labels mutual exclusivity note, full constructor and add_metric tables for
GaugeMetricFamily, CounterMetricFamily, SummaryMetricFamily,
HistogramMetricFamily, and InfoMetricFamily, plus a runnable real-world example.

collector/_index.md: constructor parameter tables for ProcessCollector,
PlatformCollector, and GCCollector, with exported metrics listed for each.

registry/_index.md (new): CollectorRegistry constructor and all public methods
(register, unregister, collect, restricted_registry, get_sample_value,
set_target_info, get_target_info), the global REGISTRY instance, and examples
for isolated registry usage and registry=None.

All code examples verified by running them in Python.

Signed-off-by: k1chik <107162115+k1chik@users.noreply.github.com>