default metrics temporality to DELTA

[trentm]

See elastic/opentelemetry#63 for discussion.
There isn't a perfect answer here. Current ES doesn't handle histograms with CUMULATIV temporality. Changing all metric types to DELTA temporality will break some existing dashboards.

Recent discussion with APM devs came to a rough concensus that EDOT agents should set the default metrics temporality to DELTA and document for users how to set it to CUMULATIVE (e.g. via OTEL_EXPORTER_OTLP_METRICS_TEMPORALITY_PREFERENCE). I think those user docs (i.e. in the EDOT Node.js docs) should include some specifics on the impact of either choice.