feat(spanner): add new open census metrics for long-running sessions removal

[harshachinta]

[DO NOT MERGE]

This PR adds a new session pool metrics using open census.
cloud.google.com/go/spanner/num_long_running_sessions_removed - The number of long-running sessions removed from the session pool.

This num_long_running_sessions_removed metric will be emitted only when ActionOnInactiveTransaction is either WarnAndClose or Close. This is because in rest of the cases there will be no change to this value and will always be 0.

Wrote unit test for testing the new metric emitted.

Real time testing
Metric dashboard - (https://screenshot.googleplex.com/9YqA4bmHoDGwsbo)

Used the below sample code to test the metric

package main

import (
	"context"
	"fmt"
	"log"
	"regexp"
	"time"

	"cloud.google.com/go/spanner"
	"google.golang.org/api/iterator"

	"contrib.go.opencensus.io/exporter/stackdriver"
)

var validDatabasePattern = regexp.MustCompile("^projects/(?P<project>[^/]+)/instances/(?P<instance>[^/]+)/databases/(?P<database>[^/]+)$")
var db = "projects/span-cloud-testing/instances/harsha-test-gcloud/databases/database1"

func main() {
	projectID, _, _, err := parseDatabaseName(db)
	if err != nil {
		log.Fatalf("Failed: %v", err)
	}

	ctx := context.Background()
	client, err := spanner.NewClientWithConfig(ctx, db, spanner.ClientConfig{
		SessionPoolConfig: spanner.SessionPoolConfig{
			MinOpened:                 0,
			MaxOpened:                 1,
			HealthCheckInterval:       1 * time.Second,
			healthCheckSampleInterval: 1 * time.Second,
			InactiveTransactionRemovalOptions: spanner.InactiveTransactionRemovalOptions{
				actionOnInactiveTransaction: spanner.WarnAndClose, // Make default to Warn from NoAction later
				executionFrequency:          1 * time.Second,
				idleTimeThreshold:           1 * time.Second,
				usedSessionsRatioThreshold:  0.95,
			},
		},
	})
	if err != nil {
		log.Fatalf("Failed: %v", err)
	}
	defer client.Close()

	// Register OpenCensus views.
	/*if err := view.Register(spanner.NumLongRunningSessionsRemovedCountView); err != nil {
		log.Fatalf("Failed: %v", err)
	}*/
	if err := spanner.EnableStatViews(); err != nil {
		log.Fatalf("Failed: %v", err)
	}

	// Create OpenCensus Stackdriver exporter.
	sd, err := stackdriver.NewExporter(stackdriver.Options{
		ProjectID: projectID,
		//ReportingInterval: 8 * time.Second,
	})
	if err != nil {
		log.Fatalf("Failed: %v", err)
	}
	// It is imperative to invoke flush before your main function exits
	defer sd.Flush()

	// Start the metrics exporter
	sd.StartMetricsExporter()
	defer sd.StopMetricsExporter()

	stmt := spanner.Statement{SQL: `SELECT SingerId, FirstName, LastName FROM Singers`}
	iter := client.Single().Query(ctx, stmt)
	for {
		row, err := iter.Next()
		time.Sleep(10 * time.Second)
		if err == iterator.Done {
			break
		}
		if err != nil {
			log.Printf("No data: %v", err)
		}
		var singerID int64
		var firstName, lastName string
		if err := row.Columns(&singerID, &firstName, &lastName); err != nil {
			log.Printf("Error: %v", err)
		}
		log.Printf("%d %s %s\n", singerID, firstName, lastName)
	}
	iter.Stop()

	iter = client.Single().Query(ctx, stmt)
	iter.Next()
	time.Sleep(10 * time.Second)
	log.Println("Main completed")
}

func parseDatabaseName(databaseUri string) (project, instance, database string, err error) {
	matches := validDatabasePattern.FindStringSubmatch(databaseUri)
	if len(matches) == 0 {
		return "", "", "", fmt.Errorf("failed to parse database name from %q according to pattern %q",
			databaseUri, validDatabasePattern.String())
	}
	return matches[1], matches[2], matches[3], nil
}

[codyoss]

@harshachinta can we close or merge this?