A newcomer to the monitoring scene is an open source tool called Prometheus. Prometheus is an open source monitoring tool that was built by a team at SoundCloud. You can find more about the project at https://prometheus.io.
Their website offers the following features:
- A multi-dimensional data model (https://prometheus.io/docs/concepts/data_model/) (the time series are identified by their metric name and key/value pairs)
- A flexible query language (https://prometheus.io/docs/prometheus/latest/querying/basics/) to leverage this dimensionality
- No reliance on distributed storage; single-server nodes are autonomous
- Time series collection happens via a pull model over HTTP
- Pushing time series (https://prometheus.io/docs/instrumenting/pushing/) is supported via an intermediary gateway
- Targets are discovered via service discovery or static configuration
- Multiple modes of graphing and dashboard support