oximeter: rework try_force_collect()

[jgallagher]

On a call today with @bnaecker and @andrewjstone discussing single-node / multi-node clickhouse, we found that the current implementation of try_force_collect() for tests is a pretty big hindrance to moving to a "both single-node and multi-node clickhouse running" world (which is where we want to be!).

The proposal we discussed was to make it a fire-and-forget action instead of the current implementation where it waits for a collection to be made. This may require reworking some tests to retry queries, but there's a reasonable chance those tests might need the same kind of rework to support querying a multi-node clickhouse in the future anyway.