What is the difference between host metrics and service metrics? – Mackerel Support

There are two main types of metrics that are monitored in Mackerel.

"Host metrics" are metrics that correspond to the host being monitored
"Service metrics" are metrics that correspond to a service that consists of multiple hosts and their collective roles

See below for more details about each type of metric.

Host Metrics

Host metrics are those that correspond to the host such as CPU and memory usage.
Host metrics are further subdivided into "system metrics" and "custom metrics".
- System metrics are posted automatically as a standard function mackerel-agent
  - Metric Specifications - Mackerel Help
- Custom metrics are posted via cloud integration or various plugins
  - AWS Integration - Mackerel Help
  - Azure Integration - Mackerel
  - Google Cloud Integration - Mackerel Help
  - Posting user-defined custom metrics - Mackerel Help
  - mackerelio/mackerel-agent-plugins: Plugins for mackerel-agent
    (※ Refer to the documentation of each cloud product / plugin for metric details)
Host metrics can also be posted from the API as well as from mackerel-agent. (※ As custom metrics)
- Host metrics（Post Metrics） - Mackerel API Documents (v0)

One of the main features of service metrics is the ability to freely post via the API.
Users must prepare their own posting mechanism, but depending on the setup, the following can be visualized and monitored.

The total number of registered users in a service
The number of PVs for a website
Business related KPIs such as sales or the number of orders received from EC sites

Response time can also be visualized as a metric by setting a threshold with an external URL monitor. (This is a standard function of Mackerel)
Check out the following API document for more on posting service metrics.

Check out the following help pages linked below to learn more about concepts like services, roles, and hosts.