Common socket connection conventions

[lmolkova]

Connection-level instrumentation is useful for all kinds of protocols to detect/investigate any connectivity issues, optimize connection pools, detect leaks, etc. The following telemetry signals are common across protocols:

• when the connection is established/terminated/closed
  • how long it takes to attempt opening a connection
  • if a connection is terminated with an error, what was the error
• connection duration and counters

The conventions could include:

• spans
  • span per attempt to connect
  • span per connection
  • higher-level spans (such as HTTP) might be able to link to them
• metrics
  • connection attempt duration
  • connection duration
  • number of active connections/connection pool size
  • number of opened connections
  • received/transmitted bytes?
  • etc

We should be able to reuse network.* and other existing common attributes to define generic connection conventions.

Individual protocols on top of sockets may extend such conventions and add protocol-specific attributes (e.g. AMQP may define some other attributes, spans/metrics).

Socket connection conventions should be reused across databases, http, messaging, and other conventions
See https://github.com/open-telemetry/semantic-conventions/blob/main/docs/database/database-metrics.md where we already define a bunch of metrics for DB connections specifically.

[trask]

this was accidentally auto-closed (via merging an old commit that closed spec issue number 454)