[APM] Only show span.sync badge when relevant

[sorenlouv]

Related: elastic/apm-agent-nodejs#1881

With #55113 we started showing badges for span.sync to indicate whether the span was async or blocking.

Problem
It is visually very noisy to display the badge for every span. This was raised in elastic/apm-agent-nodejs#1881

Solution
The badge should only be displayed when span.sync is operating in a non-default mode relative to the language in question. So for javascript the default is async so the badge should only be shown when blocking. Opposite for php.

We should talk to agent developers to get a list of the default span.sync values that we (unfortunately) have to hardcode in the ui.

[elasticmachine]

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

[cauemarcondes]

tests ok:
only blocking should be shown for nodejs.