Document how to setup datadog agent for metrics and tracing

[dasormeter]

Summary

Add a chapter in this document documenting how to setup Canton for metrics and tracing with datadog: https://github.com/digital-asset/docs.daml.com/blob/main/docs/2.10.0/docs/canton/usermanual/monitoring.rst

Need to write documentation with example canton configuration for datadog agent support

• Pointers for gathering metrics
• Interested especially in the open tracing support

Background context

for canton 2.x, we should add some documentation notes that the datadog auto-discovery annotations should be added to scrape metrics / enable open telemetry if running with kubernetes
• for datadog agent installation with kubernetes, it is effectively follow the datadog instructions to install the operator, and additional manual step is to setup the main datadog key for the operator
• for datadog agent installation w/o kubernetes, probably can just point to the datadog docs (edited)

for canton 3.x, to enable the canton components so that datadog will scrape their metrics (and to enable opentelemetry), working on some changes to the CN helm charts so that those annotations can be enabled through config