[Feature] PHP extension supports link tracking sampling rate configuration

[elricli]

Description

Like:

• skywalking.sample_n_per_3_secs in the SkyAPM/SkyAPM-php-sdk v4
• agent.sampler in the skywalking-go

Use case

Added a php.ini configuration: skywalking_agent.sampler (Sampling rate of tracing data, which is a floating-point value that must be between 0 and 1.)

Related issues

#12043

[jmjoy]

I haven't looked at the Go implementation, but I have a question:

Currently, various languages agent don't uniformly apply the first sample field of the sw8 header of the Cross Process Propagation Header. If the sampling rate is turned on and the Context is not sampled, the following problems may occur:

1. If Context is a user portal, the called Context mistakenly becomes the user portal.
2. If Context is called, the trace will be missing or broken into two segments.

Because of these reasons, I don’t know how to implement sampler better.

[wu-sheng]

@jmjoy The sampler provided above is not about the sw8 header compliance, that field is not used.
The feature mentioned above is when you can't read header, which means this is the first service in the whole distributed chain, you would use that sampler mechanism. Otherwise, once the header exists, the agent does force sampling by ignoring this sampling mechanism.

Is this clear to you?

[jmjoy]

1. If Context is a user portal, the called Context mistakenly becomes the user portal.

So this situation is possible?

[wu-sheng]

1. If Context is a user portal, the called Context mistakenly becomes the user portal.

So this situation is possible?

There is not a context called that.
This is not the agent side concern.

[heyanlong]

Based as my know, the feature just works first service, if current server can't read sw8 header then it's first service