fix(telemetry): flush spanmetrics every 15s so Cloud rate panels render

Span-derived metrics (span_calls_total, span_duration_milliseconds_*) were
emitted at the spanmetrics connector default of 60s. Grafana Cloud stores one
datapoint per flush, but dashboard panels use rate(...[$__rate_interval]),
whose window at short ranges (<=1h) falls below 60s — leaving <2 samples per
window, so those panels rendered "No data" on the Cloud datasource. (Local
Prometheus scrapes at 15s, so local dashboards were unaffected.)

Set metrics_flush_interval: 15s so every rate window holds >=2 samples at any
dashboard range. Trade-off: ~4x span-metric datapoints pushed to Cloud. Native
insight metrics (ledgermaster_*, jobq_*, peer_quality) already push faster and
were never affected.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

docker/telemetry/otel-collector-config.grafanacloud.yaml

@@ -127,6 +127,13 @@ processors:
 connectors:
   spanmetrics:
     namespace: "span"
+    # Emit span-derived metrics every 15s (default is 60s). Grafana Cloud
+    # ingests these as one datapoint per flush, and dashboard rate() panels
+    # use [$__rate_interval], which at short ranges (<=1h) can be shorter than
+    # the flush interval. A 60s flush left <2 samples per rate window on Cloud,
+    # so those panels rendered "No data". 15s keeps >=2 samples per window at
+    # every dashboard range, at the cost of ~4x span-metric datapoints to Cloud.
+    metrics_flush_interval: 15s
     # Resource attributes that define the resource-metrics grouping key.
     # All resource attributes propagate to the output metrics regardless;
     # label promotion is handled by resource_to_telemetry_conversion on the