trace_call() wraps every Spanner operation with a bare MetricsCapture()
that creates a MetricsTracer without project_id or instance_id. Since
every caller of trace_call already provides its own MetricsCapture with
resource_info, this inner one is redundant.

The redundant tracer records operation metrics with incomplete resource
labels on every operation. Because OpenTelemetry uses cumulative
aggregation, these orphan data points persist for the process lifetime
and get re-exported every 60 seconds. Cloud Monitoring rejects them
with INVALID_ARGUMENT (missing instance_id), producing repeated error
logs.

Removing the bare MetricsCapture from trace_call eliminates the orphan
metric data points entirely. Callers continue to provide their own
MetricsCapture(resource_info) with correct labels.

Fixes: googleapis#1319