According to Prometheus data model specifications, a valid metric name must adhere to the regular expression [a-zA-Z][a-zA-Z0-9]. This means the name must begin with an ASCII letter or an underscore, followed by any combination of ASCII letters, numbers, or underscores. The name gogoroutines perfectly fits this requirement. It starts with a letter and uses an underscore, which is the conventional separator for multi-word metric names, making it a valid and well-formed metric name.

A. go routines: This is incorrect because metric names cannot contain spaces.

B. go.goroutines: This is incorrect because dots (.) are not permitted characters in Prometheus metric names.

D. 99goroutines: This is incorrect because a metric name cannot start with a number; it must begin with a letter or an underscore.

1. Prometheus Official Documentation, "Metric names and labels": This section explicitly defines the valid characters and structure for metric names. It states, "Metric names ... must match the regex [a-zA-Z][a-zA-Z0-9]." This rule directly invalidates options A, B, and D, while confirming C is valid.

Source: Prometheus Authors. (n.d.). Data model. Prometheus Documentation. Retrieved from https://prometheus.io/docs/concepts/datamodel/#metric-names-and-labels

2. Prometheus Official Documentation, "Naming best practices": This guide provides conventions for naming metrics and explicitly uses gogoroutines as an example of a standard metric from the Go client library. It reinforces that underscores should be used as separators, not other characters like dots.

Source: Prometheus Authors. (n.d.). Metric and label naming. Prometheus Documentation. Retrieved from https://prometheus.io/docs/practices/naming/#metric-names

3. OpenMetrics Specification, "Syntax": OpenMetrics, the successor to the Prometheus exposition format, formally defines the syntax for metric names. The ABNF (Augmented Backus–Naur form) for a metric name is ( ALPHA / "" ) ( ALPHA / DIGIT / "" ). This confirms that names must start with a letter or underscore and can only contain letters, digits, and underscores.

Source: OpenMetrics Authors. (2023). OpenMetrics Specification, Version 1.0.0. Section: "Overall Structure". Retrieved from https://github.com/OpenObservability/OpenMetrics/blob/main/specification/OpenMetrics.md#overall-structure