feat: declare seconds unit on duration histograms

[GregTheGreek]

Summary

Adds metric.WithUnit("s") to the four Float64Histograms in MpcMetrics:

• relayer.SessionTime (PR feat: session time metrics #143)
• relayer.InitiateTime (PR feat: instrument mpc signing phase durations #171)
• relayer.CommSendTime (PR feat: instrument mpc signing phase durations #171)
• relayer.CommDnsResolveTime (PR feat: instrument mpc signing phase durations #171)

Why

sygma-core registers a sub-second bucket view in observability.InitMetricProvider:

// observability/metrics.go (initSecondView)
sdkmetric.NewView(
    sdkmetric.Instrument{Unit: "s"},
    sdkmetric.Stream{Aggregation: aggregation.ExplicitBucketHistogram{
        Boundaries: []float64{1e-6, 1e-5, 1e-4, 1e-3, 0.005, 0.01, 0.05, 0.1, 0.5, 1, 5, 10, 100, 1000, 10000},
    }},
)

The view is keyed by Instrument{Unit: "s"}. The histograms in metrics/mpc.go declared a description but no unit, so the view never matched and the SDK fell back to OTel's default histogram boundaries [5, 10, 25, 50, ..., 10000] — which are tuned for milliseconds. Signing-phase durations (sub-second to a few seconds) collapse into the le=5 bucket, making histogram_quantile return values pinned to bucket boundaries rather than real percentiles.

Values are already recorded in seconds via d.Seconds(), so no math changes — only bucketing.

Grafana query change

The OTLP→Prometheus exporter appends a _seconds suffix when the instrument carries Unit: "s". After this PR, dashboard queries change:

Before	After
relayer_SessionTime_bucket	relayer_SessionTime_seconds_bucket
relayer_InitiateTime_bucket	relayer_InitiateTime_seconds_bucket
relayer_CommSendTime_bucket	relayer_CommSendTime_seconds_bucket
relayer_CommDnsResolveTime_bucket	relayer_CommDnsResolveTime_seconds_bucket

In practice no dashboards depend on the old names yet — the OTel collector URL was only wired into staging in #174, so historical data is empty.

Test plan

• go build ./... clean
• go test ./metrics/... ./tss/... ./comm/p2p/... pass
• After deploy, confirm relayer_SessionTime_seconds_count and the three new _seconds_count series are non-empty in Grafana
• Confirm histogram_quantile(0.95, ...) returns values that vary with workload rather than snapping to bucket boundaries

[github-actions]

Go Test coverage is 37.0 %\ ✨ ✨ ✨

[github-actions]

Go Test coverage is 37.0 %\ ✨ ✨ ✨