OpenTelemetry is primarily definitions of protocols, APIs and semantic conventions for instrumentation data (traces, metric and logs, in decreasing order of maturity). OpenTelemetry also ships the Collector, which is like a patch-bay accepting many different data formats and sending on in more formats.

Grafana Agent is a bundling of a bunch of things for collecting instrumentation. The idea is to simplify deployment and allow opinionated setup. The Agent can upload to Grafana Cloud, or to any compatible backend (all the basic components are open source).

In particular it bundles Prometheus Agent, which does metrics collection from anything Prometheus-compatible but not queries, and OTel Collector.

It also bundles Promtail for logs.

(I work for Grafana Labs)

Prometheus: metrics collector/databases with a defined metrics format.

Many metrics/timeseries databases make an effort to be "Prometheus-compatible" as it was kind of the unofficial standard.

OTEL: new open standards which are supposed to provide vendor-independent formats which are compatible and composable between metrics, logs, and traces, and easily enable things like deriving metrics from traces (latency would be an obvious one here).

Grafana Agent: a metrics? maybe also logs and traces? collector that supports Prometheus, OTEL and other metrics formats, which can forward, sample, and transform the data. Made by Grafana, open source, etc.

Grafana's metrics DB Mimir and maybe some others are essentially "more scalable prometheus", and use prometheus metrics format on disc, so one large concern of the Grafana Agent would be converting OTEL metrics to prometheus metrics format for ingestion into the Grafana databases - but the agent has a whole bunch of other functions supported as well.

OTEL Collector - non-vendor-specific collector, like Grafana Agent but largely just concerned with allowing collection/ingestion of OTEL and coverting other formats into OTEL. Allows extensions and plugins to be added for other purposes.

It's a standardised way to send metrics to Prometheus. You can have a metrics exporter for Prometheus, e.g. https://opentelemetry.io/ecosystem/registry/?s=prometheus&co...

[1] https://opentelemetry.io/ecosystem/registry/?s=prometheus&co... [2] https://opentelemetry.io/docs/specs/otel/metrics/sdk_exporte...