The pinned collector warns on every start that "spanmetrics" is a deprecated
alias for "span_metrics". Rename the connector, its pipeline references and
the prose that names it.
The derived metric names are untouched. They come from the connector's
`namespace` setting rather than its component name, so occurrences inside a
metric name such as traces_spanmetrics_calls_total are deliberately left as
they are; renaming those would break every span panel. The rename is applied
only to the bare word, never where it is joined to a metric name by
underscores.
This branch introduces the connector, so the change belongs here.