openmetrics: fix histogram buckets with le="0.0" being silently dropped

[ian28223]

What does this PR do?

Fixes OpenMetrics histogram buckets with le="0.0" being silently dropped when histogram_buckets_as_distributions is enabled.

• decumulate_histogram_buckets classified the smallest bucket as "negative domain" whenever its upper bound was not > 0. For le="0.0" (upper bound == 0), this gave it lower_bound = -Inf instead of 0.
• The agent's sketch aggregation silently discards any bucket with an infinite bound, so that bucket's entire count vanished from the distribution metric, even though .count/.sum (submitted separately) still included it.
• Also relaxed the "equal bounds" skip guard in the histogram transformer so it only fires for the genuinely degenerate -Inf/-Inf case, not for legitimate finite single-point buckets like [0, 0] that this fix now produces.

Motivation

Customer escalation AGENT-17215: a Solr-backed OpenMetrics histogram with occasional 0ms-latency requests was undercounting its distribution metric by exactly the le="0.0" bucket's count, because that bucket was silently dropped end-to-end.

• Added test_histogram_buckets_as_distributions_with_zero_bucket, reproducing the reported histogram (le="0.0", le="5.0", le="+Inf") and asserting the zero bucket is submitted with lower_bound=0 instead of being dropped.
• ddev test datadog_checks_base -- tests/base/checks/openmetrics/test_v2/ — 211 passed, 3 skipped, no regressions.
• Manually verified on a live agent (Rocky Linux test VM) with a fake OpenMetrics endpoint serving the exact Solr-shaped histogram: trace logs show lower_bound:-inf (dropped) before the fix and lower_bound:0 (submitted) after.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Add qa/required if this PR needs QA validation, or qa/skip-qa if it does not. Exactly one of the two is required.
• If you need to backport this PR to another branch, you can add the backport/<branch-name> label to the PR and it will automatically open a backport PR once this one is merged

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evalya impact analysis

Impact analysis: 0 selected, 0 skipped (of 0 test tasks)
Publish tasks:   2 (always emitted)
Diff (4 files):
  datadog_checks_base/changelog.d/25283.fixed
  datadog_checks_base/datadog_checks/base/checks/openmetrics/v2/transformers/histogram.py
  datadog_checks_base/datadog_checks/base/checks/openmetrics/v2/utils.py
  datadog_checks_base/tests/base/checks/openmetrics/test_v2/test_transformers/test_histogram.py

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• Overall Coverage: 89.27% (+0.09%)

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Commit b543de9 compared against 99606e5.

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linux-aarch64	linux-x86_64	macos-aarch64	macos-x86_64	windows-x86_64
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Name	Version	Type	linux-aarch64	linux-x86_64	macos-aarch64	macos-x86_64	windows-x86_64
datadog_checks_base	38.3.2	Integration	🔺 +135 B	🔺 +135 B	🔺 +135 B	🔺 +135 B	🔺 +135 B

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Details

Name	Version	Type	linux-aarch64	linux-x86_64	macos-aarch64	macos-x86_64	windows-x86_64
datadog_checks_base	38.3.2	Integration	🔺 +44 B	🔺 +44 B	🔺 +44 B	🔺 +44 B	🔺 +44 B

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