vendor: update buildkit to v0.18.0-rc1

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

vendor/go.opentelemetry.io/otel/sdk/metric/internal/x/README.md

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+# Experimental Features
+
+The metric SDK contains features that have not yet stabilized in the OpenTelemetry specification.
+These features are added to the OpenTelemetry Go metric SDK prior to stabilization in the specification so that users can start experimenting with them and provide feedback.
+
+These feature may change in backwards incompatible ways as feedback is applied.
+See the [Compatibility and Stability](#compatibility-and-stability) section for more information.
+
+## Features
+
+- [Cardinality Limit](#cardinality-limit)
+- [Exemplars](#exemplars)
+
+### Cardinality Limit
+
+The cardinality limit is the hard limit on the number of metric streams that can be collected for a single instrument.
+
+This experimental feature can be enabled by setting the `OTEL_GO_X_CARDINALITY_LIMIT` environment value.
+The value must be an integer value.
+All other values are ignored.
+
+If the value set is less than or equal to `0`, no limit will be applied.
+
+#### Examples
+
+Set the cardinality limit to 2000.
+
+```console
+export OTEL_GO_X_CARDINALITY_LIMIT=2000
+```
+
+Set an infinite cardinality limit (functionally equivalent to disabling the feature).
+
+```console
+export OTEL_GO_X_CARDINALITY_LIMIT=-1
+```
+
+Disable the cardinality limit.
+
+```console
+unset OTEL_GO_X_CARDINALITY_LIMIT
+```
+
+### Exemplars
+
+A sample of measurements made may be exported directly as a set of exemplars.
+
+This experimental feature can be enabled by setting the `OTEL_GO_X_EXEMPLAR` environment variable.
+The value of must be the case-insensitive string of `"true"` to enable the feature.
+All other values are ignored.
+
+Exemplar filters are a supported.
+The exemplar filter applies to all measurements made.
+They filter these measurements, only allowing certain measurements to be passed to the underlying exemplar reservoir.
+
+To change the exemplar filter from the default `"trace_based"` filter set the `OTEL_METRICS_EXEMPLAR_FILTER` environment variable.
+The value must be the case-sensitive string defined by the [OpenTelemetry specification].
+
+- `"always_on"`: allows all measurements
+- `"always_off"`: denies all measurements
+- `"trace_based"`: allows only sampled measurements
+
+All values other than these will result in the default, `"trace_based"`, exemplar filter being used.
+
+[OpenTelemetry specification]: https://github.com/open-telemetry/opentelemetry-specification/blob/a6ca2fd484c9e76fe1d8e1c79c99f08f4745b5ee/specification/configuration/sdk-environment-variables.md#exemplar
+
+#### Examples
+
+Enable exemplars to be exported.
+
+```console
+export OTEL_GO_X_EXEMPLAR=true
+```
+
+Disable exemplars from being exported.
+
+```console
+unset OTEL_GO_X_EXEMPLAR
+```
+
+Set the exemplar filter to allow all measurements.
+
+```console
+export OTEL_METRICS_EXEMPLAR_FILTER=always_on
+```
+
+Set the exemplar filter to deny all measurements.
+
+```console
+export OTEL_METRICS_EXEMPLAR_FILTER=always_off
+```
+
+Set the exemplar filter to only allow sampled measurements.
+
+```console
+export OTEL_METRICS_EXEMPLAR_FILTER=trace_based
+```
+
+Revert to the default exemplar filter (`"trace_based"`)
+
+```console
+unset OTEL_METRICS_EXEMPLAR_FILTER
+```
+
+## Compatibility and Stability
+
+Experimental features do not fall within the scope of the OpenTelemetry Go versioning and stability [policy](../../../../VERSIONING.md).
+These features may be removed or modified in successive version releases, including patch versions.
+
+When an experimental feature is promoted to a stable feature, a migration path will be included in the changelog entry of the release.
+There is no guarantee that any environment variable feature flags that enabled the experimental feature will be supported by the stable version.
+If they are supported, they may be accompanied with a deprecation notice stating a timeline for the removal of that support.

vendor/go.opentelemetry.io/otel/sdk/metric/internal/x/x.go

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+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+// Package x contains support for OTel metric SDK experimental features.
+//
+// This package should only be used for features defined in the specification.
+// It should not be used for experiments or new project ideas.
+package x // import "go.opentelemetry.io/otel/sdk/metric/internal/x"
+
+import (
+	"os"
+	"strconv"
+	"strings"
+)
+
+var (
+	// Exemplars is an experimental feature flag that defines if exemplars
+	// should be recorded for metric data-points.
+	//
+	// To enable this feature set the OTEL_GO_X_EXEMPLAR environment variable
+	// to the case-insensitive string value of "true" (i.e. "True" and "TRUE"
+	// will also enable this).
+	Exemplars = newFeature("EXEMPLAR", func(v string) (string, bool) {
+		if strings.ToLower(v) == "true" {
+			return v, true
+		}
+		return "", false
+	})
+
+	// CardinalityLimit is an experimental feature flag that defines if
+	// cardinality limits should be applied to the recorded metric data-points.
+	//
+	// To enable this feature set the OTEL_GO_X_CARDINALITY_LIMIT environment
+	// variable to the integer limit value you want to use.
+	//
+	// Setting OTEL_GO_X_CARDINALITY_LIMIT to a value less than or equal to 0
+	// will disable the cardinality limits.
+	CardinalityLimit = newFeature("CARDINALITY_LIMIT", func(v string) (int, bool) {
+		n, err := strconv.Atoi(v)
+		if err != nil {
+			return 0, false
+		}
+		return n, true
+	})
+)
+
+// Feature is an experimental feature control flag. It provides a uniform way
+// to interact with these feature flags and parse their values.
+type Feature[T any] struct {
+	key   string
+	parse func(v string) (T, bool)
+}
+
+func newFeature[T any](suffix string, parse func(string) (T, bool)) Feature[T] {
+	const envKeyRoot = "OTEL_GO_X_"
+	return Feature[T]{
+		key:   envKeyRoot + suffix,
+		parse: parse,
+	}
+}
+
+// Key returns the environment variable key that needs to be set to enable the
+// feature.
+func (f Feature[T]) Key() string { return f.key }
+
+// Lookup returns the user configured value for the feature and true if the
+// user has enabled the feature. Otherwise, if the feature is not enabled, a
+// zero-value and false are returned.
+func (f Feature[T]) Lookup() (v T, ok bool) {
+	// https://github.com/open-telemetry/opentelemetry-specification/blob/62effed618589a0bec416a87e559c0a9d96289bb/specification/configuration/sdk-environment-variables.md#parsing-empty-value
+	//
+	// > The SDK MUST interpret an empty value of an environment variable the
+	// > same way as when the variable is unset.
+	vRaw := os.Getenv(f.key)
+	if vRaw == "" {
+		return v, ok
+	}
+	return f.parse(vRaw)
+}
+
+// Enabled returns if the feature is enabled.
+func (f Feature[T]) Enabled() bool {
+	_, ok := f.Lookup()
+	return ok
+}