fn: latency metrics for various call states (#1332)

* fn: latency metrics for various call states

This complements the API latency metrics available
on LB agent. In this case, we would like to measure
calls that have finished with the following status:
     "completed"
     "canceled"
     "timeouts"
     "errors"
     "server_busy"
and while measuring this latency, we subtract the
amount of time actual function execution took. This
is not precise, but an approximation mostly suitable
for trending.

Going forward, we could also subtract UDS wait time and/or
docker pull latency from this latency as an enhancement
to this PR.

api/runnerpool/runner_pool.go

@@ -5,6 +5,7 @@ import (
 	"crypto/tls"
 	"io"
 	"net/http"
+	"time"
 
 	"github.com/fnproject/fn/api/common"
 	"github.com/fnproject/fn/api/models"
@@ -61,4 +62,7 @@ type RunnerCall interface {
 	ResponseWriter() http.ResponseWriter
 	StdErr() io.ReadWriteCloser
 	Model() *models.Call
+	// For metrics/stats, add special accounting for time spent in customer code
+	AddUserExecutionTime(dur time.Duration)
+	GetUserExecutionTime() *time.Duration
 }