Golangci lint fixes (#195)

* Add golangci-lin as make target
* Fix: could omit type ... from declaration; it will be inferred from the right-hand side (staticcheck)
* Fix func intArrayContains is unused (unused)
* Fix: could use strings.ReplaceAll instead (staticcheck)
* Fix: could expand call to math.Pow (staticcheck)
* Fix: could use tagged switch on `...` (staticcheck)
* Fix: Error return value of `...` is not checked (errcheck)
* Fix: ineffectual assignment to err (ineffassign)
* Fix: There is no need to wait for command completion
* Add cpustat, diskstat and schedstat config
* Use slices to exclude metrics
* Replaced stringArrayContains by slices.Contains
* Replace m[k]=v loop with maps.Copy
* Use module slices from the standard library. Remove use of golang.org/x/exp/slices
* Use SplitSeq and max to modernize code

internal/metricRouter/metricCache.go

@@ -51,7 +51,7 @@ type MetricCache interface {
 }
 
 func (c *metricCache) Init(output chan lp.CCMessage, ticker mct.MultiChanTicker, wg *sync.WaitGroup, numPeriods int) error {
-	var err error = nil
+	var err error
 	c.done = make(chan bool)
 	c.wg = wg
 	c.ticker = ticker
@@ -161,8 +161,8 @@ func (c *metricCache) DeleteAggregation(name string) error {
 // is the current one, index=1 the last interval and so on. Returns and empty array if a wrong index
 // is given (negative index, index larger than configured number of total intervals, ...)
 func (c *metricCache) GetPeriod(index int) (time.Time, time.Time, []lp.CCMessage) {
-	var start time.Time = time.Now()
-	var stop time.Time = time.Now()
+	start := time.Now()
+	stop := time.Now()
 	var metrics []lp.CCMessage
 	if index >= 0 && index < c.numPeriods {
 		pindex := c.curPeriod - index