bug: mismatched bucket for TTFB histogram metrics

[dentiny]

Describe the bug

This is the histogram metrics for TTFB, but we're using duration bucket:

opendal/core/layers/otelmetrics/src/lib.rs

Line 234 in 9fe02f2

self.duration_seconds_boundaries.clone(),

Steps to Reproduce

N/A

Expected Behavior

Metrics and buckets should match.

Additional Context

No response

Are you willing to submit a PR to fix this bug?

• Yes, I would like to submit a PR.

[dosubot]

Good catch! This is confirmed as a bug. At line 236 in core/src/layers/otelmetrics.rs, the TTFB histogram is registered with duration_seconds_boundaries instead of the dedicated ttfb_boundaries:

register_f64_histogram_meter(
    meter,
    "opendal.operation.ttfb",
    metric,
    self.duration_seconds_boundaries.clone(),  // ❌ Should be ttfb_boundaries
)

The fix should simply change this to self.ttfb_boundaries.clone().

Why this matters: The TTFB buckets ([0.001, 0.01, 0.025, 0.05, 0.1, 0.2, 0.4, 0.8, 1.6, 3.2]) are optimized for initial response times with finer granularity at lower latencies, while duration buckets go up to 60s which is overkill for TTFB measurements.

Since you're willing to submit a PR, you may also want to check if the Prometheus or FastMetrics layers have the same issue.

To reply, just mention @dosu.

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