fix(OTEL): reduce high cardinality in traces and metrics (#9286)

# Which Problems Are Solved

There were multiple issues in the OpenTelemetry (OTEL) implementation
and usage for tracing and metrics, which lead to high cardinality and
potential memory leaks:
- wrongly initiated tracing interceptors
- high cardinality in traces:
  - HTTP/1.1 endpoints containing host names
- HTTP/1.1 endpoints containing object IDs like userID (e.g.
`/management/v1/users/2352839823/`)
- high amount of traces from internal processes (spooler)
- high cardinality in metrics endpoint:
  - GRPC entries containing host names
  - notification metrics containing instanceIDs and error messages

# How the Problems Are Solved

- Properly initialize the interceptors once and update them to use the
grpc stats handler (unary interceptors were deprecated).
- Remove host names from HTTP/1.1 span names and use path as default.
- Set / overwrite the uri for spans on the grpc-gateway with the uri
pattern (`/management/v1/users/{user_id}`). This is used for spans in
traces and metric entries.
- Created a new sampler which will only sample spans in the following
cases:
  - remote was already sampled
- remote was not sampled, root span is of kind `Server` and based on
fraction set in the runtime configuration
- This will prevent having a lot of spans from the spooler back ground
jobs if they were not started by a client call querying an object (e.g.
UserByID).
- Filter out host names and alike from OTEL generated metrics (using a
`view`).
- Removed instance and error messages from notification metrics.

# Additional Changes

Fixed the middleware handling for serving Console. Telemetry and
instance selection are only used for the environment.json, but not on
statically served files.

# Additional Context

- closes #8096 
- relates to #9074
- back ports to at least 2.66.x, 2.67.x and 2.68.x

internal/telemetry/tracing/google/google_tracer.go

@@ -28,7 +28,7 @@ type Tracer struct {
 }
 
 func (c *Config) NewTracer() error {
-	sampler := sdk_trace.ParentBased(sdk_trace.TraceIDRatioBased(c.Fraction))
+	sampler := otel.NewSampler(sdk_trace.TraceIDRatioBased(c.Fraction))
 	exporter, err := texporter.New(texporter.WithProjectID(c.ProjectID))
 	if err != nil {
 		return err

internal/telemetry/tracing/log/config.go

@@ -26,7 +26,7 @@ type Tracer struct {
 }
 
 func (c *Config) NewTracer() error {
-	sampler := sdk_trace.ParentBased(sdk_trace.TraceIDRatioBased(c.Fraction))
+	sampler := otel.NewSampler(sdk_trace.TraceIDRatioBased(c.Fraction))
 	exporter, err := stdout.New(stdout.WithPrettyPrint())
 	if err != nil {
 		return err

internal/telemetry/tracing/otel/config.go

@@ -6,6 +6,7 @@ import (
 
 	otlpgrpc "go.opentelemetry.io/otel/exporters/otlp/otlptrace/otlptracegrpc"
 	sdk_trace "go.opentelemetry.io/otel/sdk/trace"
+	api_trace "go.opentelemetry.io/otel/trace"
 
 	"github.com/zitadel/zitadel/internal/telemetry/tracing"
 	"github.com/zitadel/zitadel/internal/zerrors"
@@ -47,7 +48,7 @@ func FractionFromConfig(i interface{}) (float64, error) {
 }
 
 func (c *Config) NewTracer() error {
-	sampler := sdk_trace.ParentBased(sdk_trace.TraceIDRatioBased(c.Fraction))
+	sampler := NewSampler(sdk_trace.TraceIDRatioBased(c.Fraction))
 	exporter, err := otlpgrpc.New(context.Background(), otlpgrpc.WithEndpoint(c.Endpoint), otlpgrpc.WithInsecure())
 	if err != nil {
 		return err
@@ -56,3 +57,19 @@ func (c *Config) NewTracer() error {
 	tracing.T, err = NewTracer(sampler, exporter)
 	return err
 }
+
+// NewSampler returns a sampler decorator which behaves differently,
+// based on the parent of the span. If the span has no parent and is of kind server,
+// the decorated sampler is used to make sampling decision.
+// If the span has a parent, depending on whether the parent is remote and whether it
+// is sampled, one of the following samplers will apply:
+//   - remote parent sampled -> always sample
+//   - remote parent not sampled -> sample based on the decorated sampler (fraction based)
+//   - local parent sampled -> always sample
+//   - local parent not sampled -> never sample
+func NewSampler(sampler sdk_trace.Sampler) sdk_trace.Sampler {
+	return sdk_trace.ParentBased(
+		tracing.SpanKindBased(sampler, api_trace.SpanKindServer),
+		sdk_trace.WithRemoteParentNotSampled(sampler),
+	)
+}

internal/telemetry/tracing/sampler.go

@@ -0,0 +1,46 @@
+package tracing
+
+import (
+	"fmt"
+	"slices"
+
+	sdk_trace "go.opentelemetry.io/otel/sdk/trace"
+	"go.opentelemetry.io/otel/trace"
+)
+
+type spanKindSampler struct {
+	sampler sdk_trace.Sampler
+	kinds   []trace.SpanKind
+}
+
+// ShouldSample implements the [sdk_trace.Sampler] interface.
+// It will not sample any spans which do not match the configured span kinds.
+// For spans which do match, the decorated sampler is used to make the sampling decision.
+func (sk spanKindSampler) ShouldSample(p sdk_trace.SamplingParameters) sdk_trace.SamplingResult {
+	psc := trace.SpanContextFromContext(p.ParentContext)
+	if !slices.Contains(sk.kinds, p.Kind) {
+		return sdk_trace.SamplingResult{
+			Decision:   sdk_trace.Drop,
+			Tracestate: psc.TraceState(),
+		}
+	}
+	s := sk.sampler.ShouldSample(p)
+	return s
+}
+
+func (sk spanKindSampler) Description() string {
+	return fmt.Sprintf("SpanKindBased{sampler:%s,kinds:%v}",
+		sk.sampler.Description(),
+		sk.kinds,
+	)
+}
+
+// SpanKindBased returns a sampler decorator which behaves differently, based on the kind of the span.
+// If the span kind does not match one of the configured kinds, it will not be sampled.
+// If the span kind matches, the decorated sampler is used to make sampling decision.
+func SpanKindBased(sampler sdk_trace.Sampler, kinds ...trace.SpanKind) sdk_trace.Sampler {
+	return spanKindSampler{
+		sampler: sampler,
+		kinds:   kinds,
+	}
+}

internal/telemetry/tracing/sampler_test.go

@@ -0,0 +1,80 @@
+package tracing
+
+import (
+	"context"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	sdk_trace "go.opentelemetry.io/otel/sdk/trace"
+	"go.opentelemetry.io/otel/trace"
+)
+
+func TestSpanKindBased(t *testing.T) {
+	type args struct {
+		sampler sdk_trace.Sampler
+		kinds   []trace.SpanKind
+	}
+	type want struct {
+		description string
+		sampled     int
+	}
+	tests := []struct {
+		name string
+		args args
+		want want
+	}{
+		{
+			"never sample, no sample",
+			args{
+				sampler: sdk_trace.NeverSample(),
+				kinds:   []trace.SpanKind{trace.SpanKindServer},
+			},
+			want{
+				description: "SpanKindBased{sampler:AlwaysOffSampler,kinds:[server]}",
+				sampled:     0,
+			},
+		},
+		{
+			"always sample, no kind, no sample",
+			args{
+				sampler: sdk_trace.AlwaysSample(),
+				kinds:   nil,
+			},
+			want{
+				description: "SpanKindBased{sampler:AlwaysOnSampler,kinds:[]}",
+				sampled:     0,
+			},
+		},
+		{
+			"always sample, 2 kinds, 2 samples",
+			args{
+				sampler: sdk_trace.AlwaysSample(),
+				kinds:   []trace.SpanKind{trace.SpanKindServer, trace.SpanKindClient},
+			},
+			want{
+				description: "SpanKindBased{sampler:AlwaysOnSampler,kinds:[server client]}",
+				sampled:     2,
+			},
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			sampler := SpanKindBased(tt.args.sampler, tt.args.kinds...)
+			assert.Equal(t, tt.want.description, sampler.Description())
+
+			p := sdk_trace.NewTracerProvider(sdk_trace.WithSampler(sampler))
+			tr := p.Tracer("test")
+
+			var sampled int
+			for i := trace.SpanKindUnspecified; i <= trace.SpanKindConsumer; i++ {
+				ctx := context.Background()
+				_, span := tr.Start(ctx, "test", trace.WithSpanKind(i))
+				if span.SpanContext().IsSampled() {
+					sampled++
+				}
+			}
+
+			assert.Equal(t, tt.want.sampled, sampled)
+		})
+	}
+}