Add appropriate span.type tags on the Datadog Spans reporter

[ivantopo]

This discussion started on #1096, thanks to @seglo for bringing it up!

Datadog might show spans differently in their UI based on the span.type tag and there is a tiny list of possibilities in their docs:

• Cache
• Custom
• DB
• Serverless function
• Web

We should figure out what are the exact values for each case and automatically add those tags in the reporter.