[agent_metrics] Fill in metadata.csv descriptions (DataDog#23571)

* Fill in metadata.csv descriptions

Populate empty descriptions for datadog.agent.python.version,
datadog.agent.running, and datadog.agent.started, sourced from
https://docs.datadoghq.com/getting_started/agent/#agent-metrics

* Refine metadata.csv descriptions

* Address review: natural language tags and uniform quoting

Author: jinh-labs

agent_metrics/metadata.csv

@@ -1,4 +1,4 @@
 metric_name,metric_type,interval,unit_name,per_unit_name,description,orientation,integration,short_name,curated_metric
-datadog.agent.python.version,gauge,,,,,0,agent_metrics,py version,
-datadog.agent.running,gauge,,,,,0,agent_metrics,running,
-datadog.agent.started,count,,,,,0,agent_metrics,started,
+datadog.agent.python.version,gauge,,,,"A value of 1, tagged with the Python version.",0,agent_metrics,py version,
+datadog.agent.running,gauge,,,,"A value of 1 if the Agent is running and reporting to Datadog, tagged with the Agent version.",0,agent_metrics,running,
+datadog.agent.started,count,,,,"A count of 1 each time the Agent starts.",0,agent_metrics,started,