[chore] Update semconvgen README (#305)

* [chore] Update semconvgen README

* Fix MD013/line-length lint errors

* Update --only arg options

---------

Co-authored-by: Tyler Yahn <MrAlias@users.noreply.github.com>

semconvgen/README.md

@@ -1,28 +1,34 @@
 # Go Semantic Convention Generator
 
-This tool is designed to generate constants in a semantic convention package for the Go API and the collector.
-It may be used by other systems, but it's primary function beyond invoking the template processor is to ensure that
-generated identifiers conform to Go's naming idiom, particularly with respect to initialisms and acronyms.
+This tool is designed to generate constants in a semantic convention package
+for the Go API and the collector.
+It may be used by other systems,
+but it's primary function beyond invoking the template processor is to ensure
+that generated identifiers conform to Go's naming idiom,
+particularly with respect to initialisms and acronyms.
 Other users may be served just as well by using the template processor directly.
 
 ## Installation
 
 ```shell
-$ go get go.opentelemetry.io/build-tools/semconvgen
+go get go.opentelemetry.io/build-tools/semconvgen
 ```
+
 ## Usage
 
 ```shell
-$ semconvgen -i <path to spec YAML> -t <path to template> -o <path to output>
+semconvgen -i <path to spec YAML> -t <path to template> -o <path to output>
 ```
 
 A full list of available options:
 
+```txt
+  -c, --container string    Container image ID (default "otel/semconvgen")
+  -f, --filename string     Filename for templated output. If not specified 'basename(inputPath).go' will be used.
+  -i, --input string        Path to semantic convention definition YAML. Should be a directory in the specification git repository.
+      --only string         Process only semantic conventions of the specified type. {span, resource, event, metric_group, metric, units, scope, attribute_group}
+  -o, --output string       Path to output target. Must be either an absolute path or relative to the repository root. If unspecified will output to a sub-directory with the name matching the version number specified via --specver flag.
+  -p, --parameters string   List of key=value pairs separated by comma. These values are fed into the template as-is.
+  -s, --specver string      Version of semantic convention to generate. Must be an existing version tag in the specification git repository.
+  -t, --template string     Template filename (default "template.j2")
 ```
-  -c, --container string   Container image ID (default "otel/semconvgen")
-  -f, --filename string    Filename for templated output. If not specified 'basename(inputPath).go' will be used.
-  -i, --input string       Path to semantic convention definition YAML. Should be a directory in the specification git repository.
-  -o, --output string      Path to output target. Must be either an absolute path or relative to the repository root. If unspecified will output to a sub-directory with the name matching the version number specified via --specver flag.
-  -s, --specver string     Version of semantic convention to generate. Must be an existing version tag in the specification git repository.
-  -t, --template string    Template filename (default "template.j2")
-```

semconvgen/generator.go

@@ -41,7 +41,7 @@ func main() {
 
 	cfg := config{}
 	flag.StringVarP(&cfg.inputPath, "input", "i", "", "Path to semantic convention definition YAML. Should be a directory in the specification git repository.")
-	flag.StringVarP(&cfg.onlyType, "only", "", "", "Process only semantic conventions of the specified type. {span,resource,event,metric,units,scope}")
+	flag.StringVarP(&cfg.onlyType, "only", "", "", "Process only semantic conventions of the specified type. {span, resource, event, metric_group, metric, units, scope, attribute_group}")
 	flag.StringVarP(&cfg.specVersion, "specver", "s", "", "Version of semantic convention to generate. Must be an existing version tag in the specification git repository.")
 	flag.StringVarP(&cfg.outputPath, "output", "o", "", "Path to output target. Must be either an absolute path or relative to the repository root. If unspecified will output to a sub-directory with the name matching the version number specified via --specver flag.")
 	flag.StringVarP(&cfg.containerImage, "container", "c", "otel/semconvgen", "Container image ID")