c6e35c8: webvitals spans now carry webvitals.navigation_type and webvitals.metric_id
attributes, so downstream consumers can segment metrics by how the page was
visited (navigate, reload, back-forward, back-forward-cache,
prerender, restore) and correlate multiple values for the same metric
instance. Metrics are also deduped by instance id (metric.id) instead of
metric name, so measurements from back/forward-cache restores — which produce a
new metric instance with a new id — are no longer dropped (#269).
