Add minimal OpenTelemetry instrumentation

[pquentin]

Relates elastic/elasticsearch-py#2435

This pull request introduces minimal OpenTelemetry instrumentation, without:

• documentation
• configuration, except to enable it (it is disabled by default)
• help for the client (to get the endpoint id and path parts)
• help from the underlying HTTP clients (to get connection details)

However, it contains the basic scaffolding and can be used to send a basic trace containing the method name to Elastic APM:

Thanks @estolfo for the help. As you know, I was heavily inspired by the Ruby implementation.

[xrmx]

OT but you probably shouldn't need mock in extras_require too

[xrmx]

Given that I'm mostly clueless about this, the python implementation resembles the ruby one and this will help to move the ball forward it LGTM.

[estolfo]

hey @pquentin I'm late to the party but this LGTM!