[7.x] [Jaeger] Add gRPC sampling endpoint (#3490)

[simitt]

Backports the following commits to 7.x:

• [Jaeger] Add gRPC sampling endpoint ([Jaeger] Add gRPC sampling endpoint #3490)

[codecov-io]

Codecov Report

Merging #3497 into 7.x will increase coverage by 0.24%.
The diff coverage is 96.38%.

@@            Coverage Diff             @@
##              7.x    #3497      +/-   ##
==========================================
+ Coverage   78.98%   79.22%   +0.24%     
==========================================
  Files         109      109              
  Lines        5724     5782      +58     
==========================================
+ Hits         4521     4581      +60     
+ Misses       1203     1201       -2     

Impacted Files	Coverage Δ
agentcfg/model.go	100.00% <ø> (+11.76%)	⬆️
beater/jaeger/common.go	78.78% <ø> (-6.07%)	⬇️
beater/jaeger/grpc.go	94.82% <94.33%> (-5.18%)	⬇️
agentcfg/fetch.go	90.00% <100.00%> (+1.42%)	⬆️
beater/api/config/agent/handler.go	95.00% <100.00%> (+0.08%)	⬆️
beater/jaeger/server.go	81.81% <100.00%> (+2.10%)	⬆️
processor/otel/consumer.go	92.83% <100.00%> (ø)
kibana/connecting_client.go	72.58% <0.00%> (+8.06%)	⬆️

[axw]

Verified with this program:

package main                                                                            
                                                                                        
import (                                                                                
        "fmt"                                                                           
        "time"                                                                          
                                                                                        
        "github.com/uber/jaeger-client-go"                                              
        "github.com/uber/jaeger-client-go/config"                                       
)                                                                                       
                                                                                        
func main() {                                                                           
        cfg, _ := config.FromEnv()                                                                                                                              
        cfg.ServiceName = "schnozz"                                                     
        cfg.Sampler.Type = "remote"                                                     
        cfg.Sampler.Param = 0.1                                                         
        cfg.Sampler.SamplingRefreshInterval = 250 * time.Millisecond                    
                                                                                        
        tracer, _, _ := cfg.NewTracer()                                                                                                                                       
        for {                                                                           
                span := tracer.StartSpan("frozzle")                                     
                fmt.Println("sampled:", span.Context().(jaeger.SpanContext).IsSampled())
                span.Finish()                                                           
                time.Sleep(100 * time.Millisecond)                                      
        }
}

Process starts out sending transactions with 10% probability, and eventually picks up the remote config. However, I did notice that the indicator remains grey, as we never mark the config as applied. Something to look into for next iteration, opened #3579.