New proxy server counting logic for agent and server

[carreter]

An initial pass at the groundwork for #358 and #273 (i.e. allowing a dynamic proxy server count on both the server and the agent in a backwards-compatible manner). Currently a draft, depends on #632. Design doc here.

Changes

New package: pkg/servercounter

This package is structured around the ServerCounter interface:

// A ServerCounter counts the number of available proxy servers.
type ServerCounter interface {
	CountServers() int
}

Currently provided are StaticServerCounter, which simply returns a stored int, and CachedServerCounter, which wraps another ServerCounter and provides catching functionality with a configurable cache expiry time.

Proxy server

This PR wires the ServerCounter into the proxy server without changing its behavior by creating a StaticServerCounter with the command-line provided count.

Proxy agent

This PR wires the ServerCounter into the proxy agent without changing its behavior by creating a StaticServerCounter with a default count of 0.

This PR also adds the respectReceivedServerCount field to the agent ClientSet struct. This field toggles whether the server count received from the proxy server should overwrite the proxy agent's server counter. Since I didn't want to change the behavior of the agent yet, this is set to true by default.

Open questions

Should CountServers() return an error?

Sometimes the ServerCounter will fail to get an updated server count. In this case, we could have CountServers() return (int, error) instead.

I opted against this because we can handle errors in the ServerCounter itself. This simplifies things on the caller side and allows for including a fallback server count in the ServerCounter.

Should ServerCounter include an UpdateCount(int) method?

It might be useful for clients to (a) temporarily override the server count or (b) request an update to any cached server counts. They could do this by calling UpdateCount() with a non-negative value to temporarily override the count or with -1 to reset the override and request a count update.

I opted against this because the client can accomplish (a) using a StaticServerCounter (downside: this overwrites the previous ServerCounter if stored in the same field), and (b) seems like it would have limited use as the ServerCounter should be completely responsible for managing the server count.

A possible workaround is to add the UpdateCount() method to just CachedServerCounter rather than the entire interface. This allows for finer control of caching while maintaining a simple API for the general case.

Should respectReceivedServerCount be true or false by default?

My take: have it true by default unless an alternative server count method is provided in the commandline args to the proxy agent. This ensures backwards compatibility with old proxy servers.

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[carreter]

@avrittrohwer for visibility

[jkh52]

/ok-to-test

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@carreter: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-apiserver-network-proxy-make-lint-master	d212046	link	true	/test pull-apiserver-network-proxy-make-lint-master

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[cheftako]

Should chat on this. During certain events with multiple KASs/Konnectivity servers we can get different server counts from different servers.

[carreter]

Yeah, @avrittrohwer also raised this issue. One potential solution would be to cache the last n received server counts and take the highest value. Ultimately this gets solved by having proxy agents count the leases themselves.