metrics: add build command duration metric

This adds a build duration metric for the build command with attributes
related to the buildx driver, the error type (if any), and which options
were used to perform the build from a subset of the options.

This also refactors some of the utility methods used by the git tool to
determine filepaths into its own separate package so they can be reused
in another place.

Also adds a test to ensure the resource is initialized correctly and
doesn't error. The otel handler logging message is suppressed on buildx
invocations so we never see the error if there's a problem with the
schema url. It's so easy to mess up the schema url when upgrading OTEL
that we need a proper test to make sure we haven't broken the
functionality.

Signed-off-by: Jonathan A. Sternberg <jonathan.sternberg@docker.com>

util/confutil/node.go

@@ -0,0 +1,34 @@
+package confutil
+
+import (
+	"crypto/rand"
+	"encoding/hex"
+	"os"
+	"path/filepath"
+	"sync"
+)
+
+var nodeIdentifierMu sync.Mutex
+
+func TryNodeIdentifier(configDir string) (out string) {
+	nodeIdentifierMu.Lock()
+	defer nodeIdentifierMu.Unlock()
+	sessionFile := filepath.Join(configDir, ".buildNodeID")
+	if _, err := os.Lstat(sessionFile); err != nil {
+		if os.IsNotExist(err) { // create a new file with stored randomness
+			b := make([]byte, 8)
+			if _, err := rand.Read(b); err != nil {
+				return out
+			}
+			if err := os.WriteFile(sessionFile, []byte(hex.EncodeToString(b)), 0600); err != nil {
+				return out
+			}
+		}
+	}
+
+	dt, err := os.ReadFile(sessionFile)
+	if err == nil {
+		return string(dt)
+	}
+	return
+}