[cmd/telemetrygen] expose the generated span duration as a command pa…

[jmsnll]

Description:

As originally proposed in #26991 before I got distracted

Exposes the duration of generated spans as a command line parameter. It uses a DurationVar flag so units can be easily provided and are automatically applied.

Example usage:

telemetrygen traces --traces 100 --otlp-insecure --span-duration 10ns # nanoseconds
telemetrygen traces --traces 100 --otlp-insecure --span-duration 10us # microseconds
telemetrygen traces --traces 100 --otlp-insecure --span-duration 10ms # milliseconds
telemetrygen traces --traces 100 --otlp-insecure --span-duration 10s # seconds

Testing:

Ran without the argument provided telemetrygen traces --traces 1 --otlp-insecure and seen spans publishing with the default value.

Ran again with the argument provided: telemetrygen traces --traces 1 --otlp-insecure --span-duration 1s

And observed the expected output:

Resource SchemaURL: https://opentelemetry.io/schemas/1.4.0
Resource attributes:
     -> service.name: Str(telemetrygen)
ScopeSpans #0
ScopeSpans SchemaURL: 
InstrumentationScope telemetrygen 
Span #0
    Trace ID       : 8b441587ffa5820688b87a6b511d634c
    Parent ID      : 39faad428638791b
    ID             : 88f0886894bd4ee2
    Name           : okey-dokey
    Kind           : Server
    Start time     : 2023-11-12 02:05:07.97443 +0000 UTC
    End time       : 2023-11-12 02:05:08.97443 +0000 UTC
    Status code    : Unset
    Status message : 
Attributes:
     -> net.peer.ip: Str(1.2.3.4)
     -> peer.service: Str(telemetrygen-client)
Span #1
    Trace ID       : 8b441587ffa5820688b87a6b511d634c
    Parent ID      : 
    ID             : 39faad428638791b
    Name           : lets-go
    Kind           : Client
    Start time     : 2023-11-12 02:05:07.97443 +0000 UTC
    End time       : 2023-11-12 02:05:08.97443 +0000 UTC
    Status code    : Unset
    Status message : 
Attributes:
     -> net.peer.ip: Str(1.2.3.4)
     -> peer.service: Str(telemetrygen-server)
	{"kind": "exporter", "data_type": "traces", "name": "debug"}

Documentation: No documentation added.