[APM] Service overview: Only show data from default (or top) transaction type

[sorenlouv]

• The new service overview page won't have a transaction type selector.
• To avoid mixing data between transaction types we need to select a single type automatically
• If no transaction type is specified in the url we should show data from the default transaction type. The default transaction type for backend agents is request and page-load for RUM.
• Some services might only have custom transaction types (no default transaction types) and in these cases we should show pick the transaction type with the most transactions (top bucket sorted by number of transactions)

Thoughts on implementation
I see two approaches:

1. fetching transaction type data and perform redirect inside the service overview page
2. fetching transaction type data on service inventory page and linking to service overview page with the type in the url

Related: elastic/apm#362 (comment)

[elasticmachine]

Pinging @elastic/apm-ui (Team:apm)

[formgeist]

Just to clarify, this shouldn't change the current behavior on the Transaction views, where there's a transaction type filter option to switch between types.

[smith]

This is not the case for the error rate chart and errors table at this time so those need to be updated.

[smith]

Current status:

• Latency chart (in open PR)
• Traffic chart
• Transactions table: Does not pass in transaction type or filter by it on the server
• Error rate chart: Taking transactionType from url params not service context; does do filtering on the server
• Errors table: Does not use transaction type in client or server requests
• Average duration by span type chart
• Dependencies table: We can't filter by transaction type here because span metrics do not have a transaction type
• Instances latency distribution: Not done yet
• Instances table: Not done yet