Support periodic exporting MetricReader config from env vars

[gaborcsardi]

According to the specs [1] the periodic metric reader should read configuration from environment variables. opentelemetry-cpp does not use these environment variables currently [2]

[1] https://opentelemetry.io/docs/specs/otel/configuration/sdk-environment-variables/#periodic-exporting-metricreader
[2]

opentelemetry-cpp/sdk/include/opentelemetry/sdk/metrics/export/periodic_exporting_metric_reader_options.h

Lines 16 to 17 in f4897b2

	constexpr std::chrono::milliseconds kExportIntervalMillis = std::chrono::milliseconds(60000);
	constexpr std::chrono::milliseconds kExportTimeOutMillis = std::chrono::milliseconds(30000);