promtool: Add ability to "dump" whole blocks

Proposal

We can dump data from Prometheus with promtool tsdb dump, but unfortunately we can only import it back in OpenMetrics format so we can't re-use this file to import that data into another Prometheus server. The proposal is to add support for creating Prometheus compatible blocks from the same format that we currently dump, so we can move data from different prometheus easily.

Use case. Why is this important?
Where I work, a decision was made in the past that we'd have one Prometheus server for each application/exporter and it's not scaling too well. We'd like to consolidate all that data into a single Prometheus that will now scrape all our metrics endpoints. We want to shut down several Prometheus, but we'd like to keep their data in our new centralized one.

With this proposal, it will be a lot easier to dump and import data from one prometheus instance to another.

Start at the promtool tsdb dump entry point and trace how its output is imported through OpenMetrics. Define how that dump format can produce Prometheus-compatible blocks for another server. Done means data can be dumped from one Prometheus instance and imported into another without requiring OpenMetrics conversion.