Update OpenTelemetryEcto attributes to conform with semantic conventions 1.27

[danschultzer]

Stacked on #429 which needs to get in first. Check the latest commit for the changes here.

This conforms the attributes and span names to the 1.27 docs: https://github.com/open-telemetry/semantic-conventions/blob/v1.27.0/docs/database/database-spans.md

It does introduce a bunch of breaking changes so let me know if backwards compability is required. I can keep the old attributes, but not sure if we should just go with major release instead since there's a lot of things that are different now?

I can also update the changelog in this PR.

Changes

• db.name/db.instance -> db.namespace
• source -> db.collection.name
• db.statement -> db.query.text
• db.url -> server.address/ server.port
• total_time_ -> db.client.operation.duration (in seconds as float)
• idle_time_ -> db.client.connection.create_time (in seconds as float)
• queue_time_ -> db.client.connection.wait_time (in seconds as float)
• query_time_ -> db.client.connection.use_time (in seconds as float)
• The : db_statement config option to :db_query (not necessary but it conforms closer to the otel attribute name)
• Span name changed from ecto.repo.query to {db.operation.name} {target} or just {target} if it is a general db operation conforming to 1.27 guidelines

Removes

• db.type
• decode_time_ (I couldn't find a metric that matched it)
• The :span_prefix config option
• The :time_unit config option

Adds

• db.operation.name

Resolves #364