Allow repeated OpenMetrics metric families

[deepakganesh78]

Fixes #1091

Reproduction

Parsing OpenMetrics text shaped like promtool tsdb dump-openmetrics, where the same metric family appears in separate blocks for different targets, currently raises:

ValueError: Clashing name: go_gc_duration_seconds

Minimal shape:

# TYPE go_gc_duration_seconds summary
go_gc_duration_seconds{instance="a",quantile="0"} 1
go_gc_duration_seconds_sum{instance="a"} 1
go_gc_duration_seconds_count{instance="a"} 1
# TYPE up gauge
up{instance="a"} 1
# TYPE go_gc_duration_seconds summary
go_gc_duration_seconds{instance="b",quantile="0"} 2
go_gc_duration_seconds_sum{instance="b"} 2
go_gc_duration_seconds_count{instance="b"} 1
# EOF

Root cause

The OpenMetrics parser kept a flat set of all sample names emitted by already-built metric families. When the same family name appeared again later, its own valid sample names were treated as clashing, even though they belonged to the same family.

Fix

Track the owning metric family for each seen sample name. Repeated blocks for the same family are accepted, while real suffix collisions between different families (for example a_created versus counter a) still raise ValueError. The parser also rejects repeated family blocks if the family metadata changes type or unit.

Compatibility notes

The parser docstring says it is intentionally laxer than the main Go parser. This remains compatible with the OpenMetrics model of MetricFamilies while allowing non-contiguous family blocks such as promtool tsdb dump-openmetrics output; true name clashes are still rejected. See the OpenMetrics specification: https://github.com/OpenObservability/OpenMetrics/blob/main/specification/OpenMetrics.md

Validation

• Reproduced on current master before the fix: minimal script raised ValueError: Clashing name: go_gc_duration_seconds.
• Regression test without the parser fix: tests/openmetrics/test_parser.py::TestParse::test_repeated_metric_family failed with the same ValueError (1 failed).
• Targeted parser tests: 58 passed.
• Full suite: 427 passed, 3 skipped.
• flake8 prometheus_client/openmetrics/parser.py tests/openmetrics/test_parser.py: passed.
• isort --check prometheus_client/openmetrics/parser.py tests/openmetrics/test_parser.py: passed.
• mypy --install-types --non-interactive prometheus_client/openmetrics/parser.py tests/openmetrics/test_parser.py: reported existing Windows-only os.getpagesize/os.sysconf attribute errors in prometheus_client/process_collector.py (2 errors), unrelated to this change.