Something wrong with relabel match regexp | not working in some cases

[itshikanov]

Describe the bug
hello.
relabel match not working in some cases

To Reproduce
nginx:

log_format syslog'$host\t$status\t$upstream_addr\t$request_method\t$upstream_response_time\t$request_time\t$request_length\t$body_bytes_sent\t$request';

access_log syslog:server=127.0.0.1:8887,tag=nginx syslog;

nginxlog-exporter config:

listen {
  port = 9102
}

namespace "nginx" {
  source = {
    syslog {
      listen_address = "udp://127.0.0.1:8887"
      format = "auto"
      tags = ["nginx"]
    }
  }
  format = "$host\t$status\t$upstream_addr\t$request_method\t$upstream_response_time\t$request_time\t$request_length\t$body_bytes_sent\t$request"
  relabel "host" { from = "host" }

  relabel "request_uri" {
    from = "request"
    split = 2
#    separator = " "

    match "^(.*)$" {
      replacement = "$1"
    }
  }

}

got metrics like this

nginx_http_upstream_time_seconds_hist_bucket{host="test.local",request_uri="",status="200",le="+Inf"} 587

problem:

request_uri=""

But when i add ',' or '\t' character at the end of log_format all work fine.
example
nginx:

log_format syslog'$host\t$status\t$upstream_addr\t$request_method\t$upstream_response_time\t$request_time\t$request_length\t$body_bytes_sent\t$request,';

nginxlog:

  format = "$host\t$status\t$upstream_addr\t$request_method\t$upstream_response_time\t$request_time\t$request_length\t$body_bytes_sent\t$request,"

result:

nginx_http_upstream_time_seconds_hist_count{host="test.local",request_uri="/v1/test/wesqwex?a=1",status="200"} 424