Add support for FUNCTION STATS command

[chayim]

Redis FUNCTION STATS command was added in version 7.0.0 and is currently supported by the library. We should add support as documented here.

[elena-kolevska]

Starting work on this next week.

[elena-kolevska]

@redis/client-developers , @vmihailenco, @monkey92t I'd like to open a discussion about the API design of function stats.

Since function execution is blocking, we receive only a single function in the running_script field of the response.

In Redis Enterprise, though, clients connect to a proxy, which aggregates the information about all running scripts in the all_running_scripts field. The running_script field contains the function with the longest running time across all shards in the cluster.

Here are some ideas I had thought of:

Option 1:
We design the response object like so, matching the server response fields exactly:

type FunctionStats struct {
	Engines        []Engine
	RunningScript  RunningScript
	RunningScripts []RunningScript
}

This option is simple, but it carries the risk that people who don't use Redis Enterprise see RunningScripts as a suggestion in their IDEs and use it, unaware they will never get any value back. This is a pretty bad situation to be in, since the main use of this command is to identify long-running scripts that are blocking the server.
We should clarify cluster behaviour in the struct's doc comment.

Option 2

We could address the concern in Option 1 by populating the RunningScripts slice with the single running function from that shard, when not on Redis Enterprise, and with functions from all shards when on Redis Enterprise. Because the difference between RunningScript and RunningScripts wouldn't be obvious, we can rename RunningScript to LongestRunningScript, or even remove it completely.

We should clarify cluster behaviour in the struct's doc comment.

This option also prepares us for future improvements of the client, when we can implement the command's broadcasting to all shards and aggregate the results into RunningScripts

type FunctionStats struct {
	Engines        			[]Engine
	LongestRunningScript  	RunningScript
	RunningScripts 			[]RunningScript
}

Ideas?

[monkey92t]

i:

type RunningScript struct {
	Name     string
	Command  []interface{}
	Duration time.Duration
}

type FunctionStats struct {
	Engines   []Engine
	isRunning bool
	rs        RunningScript
	allrs     []RunningScript
}

func (fs *FunctionStats) Running() bool {
	return fs.isRunning
}

func (fs *FunctionStats) RunningScript() (RunningScript, bool) {
	return fs.rs, fs.isRunning
}

// Redis Enterprise
func (fs *FunctionStats) AllRunningScript() []RunningScript {
	return fs.allrs
}