[feature] Initial Prometheus metrics implementation (#2334)

* feat: Initial OTEL metrics

* docs: add metrics documentation

* fix: metrics endpoint conditional check

* feat: metrics endpoint basic auth

* fix: make metrics-auth-enabled default false

* fix: go fmt helpers.gen.go

* fix: add metric-related env vars to envparsing.sh

* fix: metrics docs

* fix: metrics related stuff in envparsing.sh

* fix: metrics docs

* chore: metrics docs wording

* fix: metrics stuff in envparsing?

* bump otel versions

---------

Co-authored-by: Tsuribori <user@acertaindebian>
Co-authored-by: Tsuribori <none@example.org>
Co-authored-by: tsmethurst <tobi.smethurst@protonmail.com>

vendor/go.opentelemetry.io/otel/sdk/metric/env.go

@@ -0,0 +1,50 @@
+// Copyright The OpenTelemetry Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package metric // import "go.opentelemetry.io/otel/sdk/metric"
+
+import (
+	"os"
+	"strconv"
+	"time"
+
+	"go.opentelemetry.io/otel/internal/global"
+)
+
+// Environment variable names.
+const (
+	// The time interval (in milliseconds) between the start of two export attempts.
+	envInterval = "OTEL_METRIC_EXPORT_INTERVAL"
+	// Maximum allowed time (in milliseconds) to export data.
+	envTimeout = "OTEL_METRIC_EXPORT_TIMEOUT"
+)
+
+// envDuration returns an environment variable's value as duration in milliseconds if it is exists,
+// or the defaultValue if the environment variable is not defined or the value is not valid.
+func envDuration(key string, defaultValue time.Duration) time.Duration {
+	v := os.Getenv(key)
+	if v == "" {
+		return defaultValue
+	}
+	d, err := strconv.Atoi(v)
+	if err != nil {
+		global.Error(err, "parse duration", "environment variable", key, "value", v)
+		return defaultValue
+	}
+	if d <= 0 {
+		global.Error(errNonPositiveDuration, "non-positive duration", "environment variable", key, "value", v)
+		return defaultValue
+	}
+	return time.Duration(d) * time.Millisecond
+}