fix(observability): idempotent metric registration (v26.06.97)

[ancongui]

Problem

MetricsRegistry cached its Prometheus collectors in per-instance dicts
(self._counters / self._histograms / self._gauges created in __init__), but
prometheus_client registers every Counter/Histogram/Gauge on the
process-global default REGISTRY keyed by metric name.

So a second MetricsRegistry — e.g. a second create_app() in one pytest process —
re-created an already-registered collector and prometheus raised:

ValueError: Duplicated timeseries in CollectorRegistry: pyfly_db_query_duration_seconds...

Downstream flymgmt services worked around this with a per-repo conftest fixture that
reset the registry between apps. This release fixes it at the source so that workaround
can be retired.

Fix

Move the collector caches to module level (_COUNTERS / _HISTOGRAMS / _GAUGES),
making get-or-create idempotent across every MetricsRegistry in the process — matching
prometheus's one-collector-per-name model.

• MetricsRegistry.__init__ keeps the _HAS_PROMETHEUS guard, drops the per-instance dicts.
• counter() / histogram() / gauge() read+write the module-level caches.
• Still uses the global default registry for /metrics exposition — production behavior
  unchanged; this only dedupes process-wide (no per-instance CollectorRegistry).
• Module-level dicts are typed (dict[str, Counter] etc.) so mypy --strict stays clean.
• Class docstring updated to state registration is process-global / idempotent.

Regression test

tests/observability/test_metrics_idempotent.py — two MetricsRegistry instances calling
.counter() / .histogram() / .gauge() with the same name do not raise and return the
same (is) collector object (the exact scenario that used to raise "Duplicated timeseries").

Two existing tests that poked the removed _counters dict now use the public .counter()
accessor (which returns the same cached collector).

Gates (all green locally)

• ruff check src/ tests/ — pass
• ruff format --check src/ tests/ — pass
• mypy src/pyfly --strict — Success: no issues found in 673 source files
• pytest tests/observability -q — 31 passed
• pytest tests/container tests/context tests/observability -q — 377 passed