To count the number of messages containing the term ERROR, which CloudWatch feature should be utilized?

To count the number of messages containing the term ERROR, utilizing a metric filter in CloudWatch is the appropriate choice. Metric filters allow you to extract metric data from the log events generated by your applications and services, enabling you to count specific patterns or occurrences within your logs. By creating a metric filter that looks for the term ERROR, you can effectively track and count how many log entries match that criterion over time.

This becomes particularly valuable when monitoring application performance and reliability, as it helps in identifying potential issues indicated by error messages. The metric filter processes log data and converts it into a format that can be viewed and analyzed through CloudWatch metrics, thus providing real-time insights into the application's health.

Other features like Composite alarms, Lambda insights, and Synthetics canary do not directly perform log analysis for counting specific terms. Composite alarms are used to combine multiple alarms to create a single alarm state, Lambda insights provides performance monitoring insights for Lambda functions, and Synthetics canary is used to monitor the availability and performance of web applications through scripted interactions. None of these functions provide the capability to directly filter and quantify log entries based on specific text patterns like a metric filter does.