<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->
#### Description

This is another fix of open-telemetry#43044. Probably one more PR will be needed!

Addressed 2 of the benchmark failures in `pkg/stanza/adapter` here.
 
```
--- FAIL: BenchmarkReadLine
    receiver_test.go:285: 
                Error Trace:    /Users/ray.kang/github.com/gnak-yar/opentelemetry-collector-contrib/pkg/stanza/adapter/receiver_test.go:285
                Error:          Received unexpected error:
                                '' unsupported type 'file_input'
                Test:           BenchmarkReadLine
--- FAIL: BenchmarkParseAndMap
    receiver_test.go:369: 
                Error Trace:    /Users/ray.kang/github.com/gnak-yar/opentelemetry-collector-contrib/pkg/stanza/adapter/receiver_test.go:369
                Error:          Received unexpected error:
                                unsupported type 'file_input'
                Test:           BenchmarkParseAndMap
```


**BenchmarkReadLine**

The `file_input` operator should be registered to a global operator
registry before run. The registration is done when the file input
operator package is imported, but the package isn't imported so the
error occurs.

I've removed the benchmark instead of fixing it. Here's the reason.
- As far as I understand, this benchmark is to see the `file_input`
operator's performance, but we have other benchmarks doing almost the
same thing.
-
[file.BenchmarkReadExistingLogs](https://github.com/open-telemetry/opentelemetry-collector-contrib/blob/7e4bf11279e5b454980626237ccd1417b7e0e26a/pkg/stanza/operator/input/file/benchmark_test.go#L23)
-
[fileconsumer.BenchmarkFileInput](https://github.com/open-telemetry/opentelemetry-collector-contrib/blob/7e4bf11279e5b454980626237ccd1417b7e0e26a/pkg/stanza/fileconsumer/benchmark_test.go#L31)
- Even the test seems a bit weird to me because the `b.N` loop(`for i :=
0; i < b.N; i++ {`) is used for log generation, not for a function to be
measured.

**BenchmarkParseAndMap**

Two reasons of the failure
- The `file_input` package is not imported as well.
- Unmarshaling yaml configs(line 369 before change) does not work well,
as the configs are not supposed to be loaded from yaml directly. For
example, the following structure is a part of `fileconsumer.Config`.
`yaml.Unmarshaler` does not understand the embeded fields by
default(Probably a more tag like `yaml:",inline"` is required).
    ```go
    type Config struct {
        matcher.Criteria        `mapstructure:",squash"`
        attrs.Resolver          `mapstructure:",squash"`
        ...
    }
    ```

As far as I understand, `BenchmarkParseAndMap` is to see the performance
of `regex_parser`'s severity mappings. So, I've moved the benchmark to
`regex_parser` and made it focus on the operator's performance.


I'm not familiar with the full history and background. Please let me
know if you have any concerns or suggestions!

<!-- Issue number (e.g. open-telemetry#1234) or full URL to issue, if applicable. -->
#### Link to tracking issue
Fixes
open-telemetry#43044 

<!--Describe what testing was performed and which tests were added.-->
#### Testing
```
cd pkg/stanza/adapter
go test -bench='^Benchmark(ReadLine|ParseAndMap)$'

# For testing a benchmark moved to the regex parser.
cd pkg/stanza/operator/parser/regex
go test -bench='^BenchmarkProcessBatch'
```

---------

Co-authored-by: Andrzej Stencel <[email protected]>