Parse negative span offsets in OpenMetrics native histograms - #1233

The span regex in _parse_nh_struct only matched unsigned offsets, so a span list such as positive_spans:[-2:2] was dropped and the parsed NativeHistogram ended up with deltas but no spans. The first span offset is the starting bucket index and is negative for buckets below 1, which is common for histograms in seconds. The client's own OpenMetrics exposition writes such offsets, so its output did not round trip.

This allows an optional minus sign on span offsets, matching what the deltas regex already does, and adds a parser test with negative offsets in both positive and negative spans.

@csmarchbanks

Fixes #1232