Fix datadog_metrics handling of Prometheus +Inf histogram buckets

[anvithsg2004]

What does this PR do?

This fixes an issue where Prometheus histogram observations that fall into the implicit +Inf bucket can be lost by the datadog_metrics sink and result in an empty Datadog sketch payload.

The Prometheus parser previously removed the +Inf bucket after converting cumulative bucket counts to per-bucket counts. This caused valid observations above the highest finite bucket to be discarded.

This PR:

• Preserves the +Inf bucket and its observations in the Prometheus parser.
• Handles the edge case where a histogram contains only a +Inf bucket during DDSketch interpolation.
• Filters empty sketches after histogram/distribution conversion so empty Datadog sketch payloads are not sent.
• Filters already-empty Sketch metrics as well.

Why?

For example, a histogram such as:

test_overflow_bucket{le="1.0"} 0
test_overflow_bucket{le="+Inf"} 4
test_overflow_count 4
test_overflow_sum 20000

represents four valid observations above 1.0.

Previously, the +Inf bucket was removed, leaving no bucket observations to insert into the DDSketch. This could produce an empty sketch, which was subsequently encoded as an empty SketchPayload and rejected by Datadog with HTTP 400 Bad Request.

The +Inf bucket is now preserved, allowing the observations to be represented in the DDSketch.

Tests

Added and updated tests covering:

• Prometheus histograms containing +Inf buckets.
• DDSketch conversion with finite and +Inf buckets.
• Histograms containing only a +Inf bucket.
• Filtering of empty sketches and aggregated histograms.
• Successful normalization of histograms containing +Inf observations.

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[anvithsg2004]

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[chatgpt-codex-connector]

Set the backward-histogram +Inf expectation correctly

In test_histogram_backward_values, the reverse conversion loop processes the +Inf bucket before mutating the 10 bucket, so its count becomes 2000 - 1000 = 1000; it does not become zero. Consequently, this changed expectation makes the parser unit test fail deterministically. Expect f64::INFINITY => 1000, or change the conversion behavior and test input consistently.

AGENTS.md reference: AGENTS.md:L62-L66

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