Introduce generic operation.name attribute to represent span name

[trask]

(and rename db.operation.name to db.operation.command and replace db.query.summary with operation.name)

Introducing a generic operation.name attribute for span name would allow metrics to stamp this generally useful, low cardinality attribute onto metrics. This would also benefit span to metrics pipelines to have a standard place to put the span name.

It would also provide a way for users to override the span name using something like open-telemetry/oteps#207, which is important for SQL queries which are going to have pretty bad span names unless languages implement complex SQL parsing, and even then, they won't have great span names).

Overriding the span name has also been a highly requested feature for HTTP client spans, where the default span names are also pretty bad.

In database metrics, by replacing db.query.summary with the more generic operation.name, users would get a consistent database metric drilldown experience across both sql and nosql databases, which is currently a bit lacking because operation and collection name are typically only captured on nosql databases (see #1566).