Prometheus compatibility: use target_info metric instead of "target" info MF

[dashpole]

Background

There was some confusion around the use of "target" info MetricFamily vs target_info Metric, which even caused me to incorrectly implement the feature in the collector.

In OpenMetrics, MetricFamily refers to the group of multiple metric datapoints which share the same name. For example, a histogram MetricFamily named scrape_duration_seconds contains Metrics named scrape_duration_seconds_sum and scrape_duration_seconds_count, etc. The suffixes appended to the Metric(s) within a MetricFamily depend on the metric type. For info-typed metrics, the suffix is always _info. Thus, the target info-typed MetricFamily has Metric(s) named target_info:

# TYPE target info
target_info{service_name="my-service",service_instance_id="123456"} 1

In Prometheus (not OpenMetrics), "info" is a convention, rather than a type, and are thus represented with a gauge, which has no type suffix. Thus, in prometheus, both the MetricFamily and the Metric(s) are named target_info.

# TYPE target_info gauge
target_info{service_name="my-service",service_instance_id="123456"} 1

Changes

Since the common portion between OM and Prom is the metric name, rather than the metric family, update references in the spec for "target" info MetricFamily to target_info metric.

cc @jmacd