Add ability to clear history for individual metrics

[juliusv]

From @mweiden:

"Adding the ability to clear the history for one metric instead of removing all storage via 'sudo rm -rf /srv/prometheus/storage/*' would make debugging metric design much less painful and leave other crucial data intact."

[beorn7]

#743 has added the required method to the storage. Now we need an API...

[fabxc]

We wanted to document an official v1 HTTP-API.
So we first will want to define a consistent one. Not sure whether the current endpoints fulfill that - especially in terms of the response format.

[juliusv]

At least there's nothing obvious I'd want to change about the response formats for now. We made some adjustments to the response format (#389) before the January announcement, so that we wouldn't have to change it so much anymore afterwards.

We can think if we want to amend/adjust the query parameters. Currently you provide end and range for range queries, instead of start and end. We could allow optionally specifying start instead of end, but that could be added later in a non-breaking way.

[fabxc]

Yes, was not so much about changing but about verifying that everything is consistent.
I actually started a design doc this morning - let's see.

If there's something we can do better, we shouldn't be hold back by a semi-official announcement in a buried issue.

Backwards compatibility is always an option.

[juliusv]

Keep in mind that PromDash, Grafana, and other tools already use the query API as it stands today, so even without an official API announcement there's already tons of things we'd break if we changed something in a backwards-incompatible way. Hoping for v1 of the API to be compatible with today's API in the best case...

[fabxc]

That's why I think we should just define a public API and provide the old one in parallel. We will want to do API-versioning, so the endpoint paths break anyway.

Filing two PRs for Grafana and PromDash over time to eventually switch shouldn't be the issue.

[juliusv]

This just got fixed in #783

[lock]

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