feat: add metrics_exporter for metrics_sdk

[xuan-cao-swi]

Description

Added otlp metrics exporter that works with otlp collector through http protocol.

Most code copied from (trace) exporter (with test as well) with the assumption of same ssl, http etc. configuration. The major change is the proto decode. If the log exporter also use the similar configuration, having a utility file can help DRY.

Quick demo

require 'opentelemetry/sdk'
require 'opentelemetry-metrics-sdk' # need to install this from building the metrics_sdk and metrics_api folder
require 'opentelemetry/exporter/otlp'

OpenTelemetry::SDK.configure

otlp_metric_exporter = OpenTelemetry::Exporter::OTLP::MetricsExporter.new

OpenTelemetry.meter_provider.add_metric_reader(otlp_metric_exporter)

meter = OpenTelemetry.meter_provider.meter("SAMPLE_METER_NAME")

histogram = meter.create_histogram('histogram', unit: 'smidgen', description: 'desscription')

histogram.record(123, attributes: {'foo' => 'bar'})

OpenTelemetry.meter_provider.metric_readers.each(&:pull)
OpenTelemetry.meter_provider.shutdown

Testing failed but it will success once the open-telemetry#1532 is merged.