[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/exporter.go

@@ -0,0 +1,88 @@
+// 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 (
+	"context"
+	"fmt"
+
+	"go.opentelemetry.io/otel/sdk/metric/metricdata"
+)
+
+// ErrExporterShutdown is returned if Export or Shutdown are called after an
+// Exporter has been Shutdown.
+var ErrExporterShutdown = fmt.Errorf("exporter is shutdown")
+
+// Exporter handles the delivery of metric data to external receivers. This is
+// the final component in the metric push pipeline.
+type Exporter interface {
+	// Temporality returns the Temporality to use for an instrument kind.
+	//
+	// This method needs to be concurrent safe with itself and all the other
+	// Exporter methods.
+	Temporality(InstrumentKind) metricdata.Temporality
+	// DO NOT CHANGE: any modification will not be backwards compatible and
+	// must never be done outside of a new major release.
+
+	// Aggregation returns the Aggregation to use for an instrument kind.
+	//
+	// This method needs to be concurrent safe with itself and all the other
+	// Exporter methods.
+	Aggregation(InstrumentKind) Aggregation
+	// DO NOT CHANGE: any modification will not be backwards compatible and
+	// must never be done outside of a new major release.
+
+	// Export serializes and transmits metric data to a receiver.
+	//
+	// This is called synchronously, there is no concurrency safety
+	// requirement. Because of this, it is critical that all timeouts and
+	// cancellations of the passed context be honored.
+	//
+	// All retry logic must be contained in this function. The SDK does not
+	// implement any retry logic. All errors returned by this function are
+	// considered unrecoverable and will be reported to a configured error
+	// Handler.
+	//
+	// The passed ResourceMetrics may be reused when the call completes. If an
+	// exporter needs to hold this data after it returns, it needs to make a
+	// copy.
+	Export(context.Context, *metricdata.ResourceMetrics) error
+	// DO NOT CHANGE: any modification will not be backwards compatible and
+	// must never be done outside of a new major release.
+
+	// ForceFlush flushes any metric data held by an exporter.
+	//
+	// The deadline or cancellation of the passed context must be honored. An
+	// appropriate error should be returned in these situations.
+	//
+	// This method needs to be concurrent safe.
+	ForceFlush(context.Context) error
+	// DO NOT CHANGE: any modification will not be backwards compatible and
+	// must never be done outside of a new major release.
+
+	// Shutdown flushes all metric data held by an exporter and releases any
+	// held computational resources.
+	//
+	// The deadline or cancellation of the passed context must be honored. An
+	// appropriate error should be returned in these situations.
+	//
+	// After Shutdown is called, calls to Export will perform no operation and
+	// instead will return an error indicating the shutdown state.
+	//
+	// This method needs to be concurrent safe.
+	Shutdown(context.Context) error
+	// DO NOT CHANGE: any modification will not be backwards compatible and
+	// must never be done outside of a new major release.
+}