Add Datadog tracing library

Created by: daxmc99

Part of https://github.com/sourcegraph/sourcegraph/issues/30699

This reverts commit 7c6adcdea86f367aabc9716183bd20c92e211171.

This PR adds a new config field to observability.tracing called type. It defaults to opentracing which keeps the previous behavior. In order to enable the new Datadog tracer, you need to change this config option to specific observability.tracing": { "type": "datadog" }

Test plan

Added additional docs on how to enable and disable this flag on production systems.

Previously reviewed in https://github.com/sourcegraph/sourcegraph/pull/31294