feat: add datadog timing metrics

[sky-mart]

It is required to use the datadog crate in orb-core

[github-actions]

Found one concrete issue:

• orb-dogd/src/dd.rs:218: timing uses self.tx.send(metric), unlike the existing metric methods which go through self.emit(metric) and use try_send. Since the channel is bounded and the worker waits for the Datadog socket before draining, the 513th timing metric can block indefinitely when the agent/socket is unavailable. This can also block async runtime threads if timing is emitted from async code. Use self.emit(metric) here so timing preserves the same nonblocking backpressure behavior as count, gauge, hist, and dist.

[github-actions]

Finding

• orb-dogd/src/dd.rs:218: timing uses blocking self.tx.send(metric) while every other metric path uses self.emit(metric), which calls try_send. This can block callers when the bounded channel fills. That is especially risky because the worker does not drain the channel until it has connected to the Datadog socket, so if /run/datadog/dsd.socket is absent or slow to appear, timing calls can hang after the buffer fills. Please route this through self.emit(metric) like the other methods.