Add collection tokens into oximeter

[smklein]

Attempts to improve the force_collect method on Oximeter:

omicron/oximeter/collector/src/lib.rs

Lines 550 to 556 in e69a5a3

	/// Forces Oximeter to perform a collection immediately.
	///
	/// This is particularly useful during tests, which would prefer to
	/// avoid waiting until a collection interval completes.
	pub async fn force_collect(&self) {
	self.server.app_private().force_collection().await
	}

This method triggers the collection of metrics from Producers, but Oximeter is a pipeline, and this just sends an initial message, rather than ensuring that those metrics have made their way to Clickhouse.

This PR weaves an optional "token" through this pipeline, so a caller can call force_collect and confidently know "if any metrics were ready to be produced before, they'll be in Clickhouse after this call".

Fixes #2186

[bnaecker]

Nice! Couple of spelling nits, but 👍 otherwise.