fix(cli): allow disabling debug listening ports for agent (#20671)

A customer reported unexpected port allocation in their workspace. When
looking into it I noticed we always hijack these ports and there is no 
way to disable them entirely.

This change allows the servers to be disabled by setting them to the
empty string. Previously they would still listen on ephemeral ports.

```console
❯ coder agent --help | grep -E '211[2-3]|6060'
      --debug-address string, $CODER_AGENT_DEBUG_ADDRESS (default: 127.0.0.1:2113)
      --pprof-address string, $CODER_AGENT_PPROF_ADDRESS (default: 127.0.0.1:6060)
      --prometheus-address string, $CODER_AGENT_PROMETHEUS_ADDRESS (default: 127.0.0.1:2112)
```

There are now two ways to disable, either via CLI or env variables:

```console
# Flags.
coder agent --debug-address= --pprof-address= --prometheus-address=

# Environment variables.
export CODER_AGENT_DEBUG_ADDRESS=
export CODER_AGENT_PPROF_ADDRESS=
export CODER_AGENT_PROMETHEUS_ADDRESS=
coder agent
```

cli/agent.go

@@ -11,6 +11,7 @@ import (
 	"os"
 	"path/filepath"
 	"runtime"
+	"slices"
 	"strconv"
 	"strings"
 	"time"
@@ -201,18 +202,15 @@ func workspaceAgent() *serpent.Command {
 			// Enable pprof handler
 			// This prevents the pprof import from being accidentally deleted.
 			_ = pprof.Handler
-			pprofSrvClose := ServeHandler(ctx, logger, nil, pprofAddress, "pprof")
-			defer pprofSrvClose()
-			if port, err := extractPort(pprofAddress); err == nil {
-				ignorePorts[port] = "pprof"
-			}
+			if pprofAddress != "" {
+				pprofSrvClose := ServeHandler(ctx, logger, nil, pprofAddress, "pprof")
+				defer pprofSrvClose()
 
-			if port, err := extractPort(prometheusAddress); err == nil {
-				ignorePorts[port] = "prometheus"
-			}
-
-			if port, err := extractPort(debugAddress); err == nil {
-				ignorePorts[port] = "debug"
+				if port, err := extractPort(pprofAddress); err == nil {
+					ignorePorts[port] = "pprof"
+				}
+			} else {
+				logger.Debug(ctx, "pprof address is empty, disabling pprof server")
 			}
 
 			executablePath, err := os.Executable()
@@ -276,6 +274,28 @@ func workspaceAgent() *serpent.Command {
 			for {
 				prometheusRegistry := prometheus.NewRegistry()
 
+				promHandler := agent.PrometheusMetricsHandler(prometheusRegistry, logger)
+				var serverClose []func()
+				if prometheusAddress != "" {
+					prometheusSrvClose := ServeHandler(ctx, logger, promHandler, prometheusAddress, "prometheus")
+					serverClose = append(serverClose, prometheusSrvClose)
+
+					if port, err := extractPort(prometheusAddress); err == nil {
+						ignorePorts[port] = "prometheus"
+					}
+				} else {
+					logger.Debug(ctx, "prometheus address is empty, disabling prometheus server")
+				}
+
+				if debugAddress != "" {
+					// ServerHandle depends on `agnt.HTTPDebug()`, but `agnt`
+					// depends on `ignorePorts`. Keep this if statement in sync
+					// with below.
+					if port, err := extractPort(debugAddress); err == nil {
+						ignorePorts[port] = "debug"
+					}
+				}
+
 				agnt := agent.New(agent.Options{
 					Client:        client,
 					Logger:        logger,
@@ -299,10 +319,15 @@ func workspaceAgent() *serpent.Command {
 					},
 				})
 
-				promHandler := agent.PrometheusMetricsHandler(prometheusRegistry, logger)
-				prometheusSrvClose := ServeHandler(ctx, logger, promHandler, prometheusAddress, "prometheus")
-
-				debugSrvClose := ServeHandler(ctx, logger, agnt.HTTPDebug(), debugAddress, "debug")
+				if debugAddress != "" {
+					// ServerHandle depends on `agnt.HTTPDebug()`, but `agnt`
+					// depends on `ignorePorts`. Keep this if statement in sync
+					// with above.
+					debugSrvClose := ServeHandler(ctx, logger, agnt.HTTPDebug(), debugAddress, "debug")
+					serverClose = append(serverClose, debugSrvClose)
+				} else {
+					logger.Debug(ctx, "debug address is empty, disabling debug server")
+				}
 
 				select {
 				case <-ctx.Done():
@@ -314,8 +339,11 @@ func workspaceAgent() *serpent.Command {
 				}
 
 				lastErr = agnt.Close()
-				debugSrvClose()
-				prometheusSrvClose()
+
+				slices.Reverse(serverClose)
+				for _, closeFunc := range serverClose {
+					closeFunc()
+				}
 
 				if mustExit {
 					break