Document how to interpret tracing

The [docs on tracing](https://hasura.io/docs/latest/observability/tracing/) need to help users understand what the different elements in a Hasura distributed tracing mean. [Ref](https://stackoverflow.com/c/hasura/questions/100)(incomplete info here).

Start with the tracing documentation at https://hasura.io/docs/latest/observability/tracing/ and compare it with the incomplete reference at https://stackoverflow.com/c/hasura/questions/100. Document what the different elements in Hasura distributed traces mean, with enough context for users to interpret them; the issue does not name specific files or tests.

Written by the indexing model from the issue text.