Add agent tagging / filtering for pipelines (#902)

Officially support labels for pipelines and agents to improve pipeline picking. 

* add pipeline labels
* update, improve docs  and add migration
* update proto file

---
closes #304 & #860

cmd/agent/agent.go

@@ -20,6 +20,7 @@ import (
 	"net/http"
 	"os"
 	"runtime"
+	"strings"
 	"sync"
 
 	"github.com/rs/zerolog"
@@ -39,18 +40,26 @@ import (
 )
 
 func loop(c *cli.Context) error {
-	filter := rpc.Filter{
-		Labels: map[string]string{
-			"platform": runtime.GOOS + "/" + runtime.GOARCH,
-		},
-		Expr: c.String("filter"),
-	}
-
 	hostname := c.String("hostname")
 	if len(hostname) == 0 {
 		hostname, _ = os.Hostname()
 	}
 
+	labels := map[string]string{
+		"hostname": hostname,
+		"platform": runtime.GOOS + "/" + runtime.GOARCH,
+		"repo":     "*", // allow all repos by default
+	}
+
+	for _, v := range c.StringSlice("filter-labels") {
+		parts := strings.SplitN(v, "=", 2)
+		labels[parts[0]] = parts[1]
+	}
+
+	filter := rpc.Filter{
+		Labels: labels,
+	}
+
 	if c.Bool("pretty") {
 		log.Logger = log.Output(
 			zerolog.ConsoleWriter{

cmd/agent/flags.go

@@ -72,10 +72,10 @@ var flags = []cli.Flag{
 		Name:    "hostname",
 		Usage:   "agent hostname",
 	},
-	&cli.StringFlag{
-		EnvVars: []string{"WOODPECKER_FILTER"},
+	&cli.StringSliceFlag{
+		EnvVars: []string{"WOODPECKER_FILTER_LABELS"},
 		Name:    "filter",
-		Usage:   "filter expression to restrict builds by label",
+		Usage:   "List of labels to filter tasks on. An agent must be assigned every tag listed in a task to be selected.",
 	},
 	&cli.IntFlag{
 		EnvVars: []string{"WOODPECKER_MAX_PROCS"},