test: drive prometheus_mp through nixlTelemetry (#2054)

## What?

Covers the `prometheus_mp` telemetry exporter's core path in gtest by
driving it through
`nixlTelemetry` instead of calling `exportEvent` directly: the
per-metric allowlist keeping a
deactivated metric out, the dropped-events counter accumulating every
flush delta, and
conservation of accepted + dropped events when a small staging queue
overflows (both the
single-event path and the all-or-none 4-event `addXferStats` batch).

Tests only; no production code changes.

## Why?

Everything this exporter had constructed it directly and called
`exportEvent`, which skips the
core: the staging queue, the drain task, metric gating and the drop
accounting were never
exercised for it. NIX-1685 makes `prometheus_mp` the configured exporter
everywhere, and an
exporter should not become a default while its core path is untested.

The allowlist is the case that matters most. Per-metric activation gates
producer-side, before
the staging queue, so it *should* apply identically to any exporter —
but nothing proved a
deactivated metric stays out of an mp store, and NIX-1685 would make
that a default-path
behaviour. Closes NIX-1682.

## How?

These tests need an HTTP scrape and a way to query the parsed result.
Three implementations of
that already existed: two hand-rolled ones in gtest (string matching
over the raw body) and a
proper loopback client, OpenMetrics parser and series-query layer in the
DOCA tests. Rather
than add a fourth, the DOCA one moved to `test/metrics/` as the shared
toolkit and the gtest
telemetry tests were converted onto it, which deleted more code than the
new tests add.
Waiting on an expected end state rather than sleeping a fixed interval
is what keeps the new
tests timing-independent, and it also cut suite runtime.

The mp fixtures got the same treatment: four suites each opened their
own per-test temp
directory, two of them separately repeating the mode-0700 chmod the
exporter needs in order
not to warn (a warning fails these tests), so that rule now lives in one
place. Descriptors
are owned by `nixl::scopedFd` rather than hand-rolled closers.

The allowlist test asserts on the store snapshot as well as on the
scrape: an untouched slot
proves the event never reached the exporter, rather than merely not
being rendered.

Verified with `./build/test/gtest/gtest
--gtest_filter='Mp*:*Prometheus*:*Telemetry*'` (88/88,
stable over `--gtest_repeat=5`) and the DOCA telemetry targets, which
exercise the moved
scrape toolkit.


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

- **Tests**
- Expanded multi-process telemetry coverage for metric filtering,
dropped-event accounting, overflow conservation, histogram data, and
stale-process cleanup.
- Improved validation of Prometheus metrics, labels, counters, gauges,
and histograms.
- Added shared test utilities for scraping, parsing, temporary
resources, and telemetry setup.
- **Refactor**
- Consolidated telemetry test helpers under a shared metrics-testing
framework.
- Improved socket and file-descriptor cleanup through automatic resource
management.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

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Signed-off-by: Efraim Eygin <eeygin@nvidia.com>