telemetrygen: Allow setting custom paths (#24551)

The current description for the flag is wrong.

```
❯ ../../open-telemetry/opentelemetry-collector-contrib/cmd/telemetrygen/telemetrygen metrics --otlp-endpoint example.com/v1/metrics  --otlp-http --otlp-insecure
2023-07-25T22:03:45.884+0200	INFO	metrics/metrics.go:56	starting HTTP exporter
Error: failed to obtain OTLP exporter: parse "http://example.com%2Fv1%2Fmetrics/v1/metrics": invalid URL escape "%2F"
```

I couldn't make it easily work with the existing flag because
`url.Parse(c.Endpoint)` puts everything in path, and u.Host is unset.
This is because url.Parse expects `http://` or `https://` which is set
through a different flag.

Signed-off-by: Goutham <[email protected]>
Co-authored-by: Pablo Baeyens <[email protected]>

.chloggen/telemetrygen-new-path.yaml

@@ -0,0 +1,20 @@
+# Use this changelog template to create an entry for release notes.
+# If your change doesn't affect end users, such as a test fix or a tooling change,
+# you should instead start your pull request title with [chore] or use the "Skip Changelog" label.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: enhancement
+
+# The name of the component, or a single word describing the area of concern, (e.g. filelogreceiver)
+component: cmd/telemetrygen
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: "Add ability to set custom path to endpoint."
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [24551]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:

cmd/telemetrygen/internal/common/config.go

@@ -53,6 +53,7 @@ type Config struct {
 	Endpoint           string
 	Insecure           bool
 	UseHTTP            bool
+	HTTPPath           string
 	Headers            KeyValue
 	ResourceAttributes KeyValue
 }
@@ -75,9 +76,10 @@ func (c *Config) CommonFlags(fs *pflag.FlagSet) {
 	fs.DurationVar(&c.TotalDuration, "duration", 0, "For how long to run the test")
 	fs.DurationVar(&c.ReportingInterval, "interval", 1*time.Second, "Reporting interval (default 1 second)")
 
-	fs.StringVar(&c.Endpoint, "otlp-endpoint", "localhost:4317", "Target to which the exporter is going to send metrics. This MAY be configured to include a path (e.g. example.com/v1/metrics)")
+	fs.StringVar(&c.Endpoint, "otlp-endpoint", "localhost:4317", "Target to which the exporter is going to send metrics.")
 	fs.BoolVar(&c.Insecure, "otlp-insecure", false, "Whether to enable client transport security for the exporter's grpc or http connection")
 	fs.BoolVar(&c.UseHTTP, "otlp-http", false, "Whether to use HTTP exporter rather than a gRPC one")
+	fs.StringVar(&c.HTTPPath, "otlp-http-url-path", "/v1/metrics", "Which URL path to write to (default /v1/metrics)")
 
 	// custom headers
 	c.Headers = make(map[string]string)

cmd/telemetrygen/internal/metrics/metrics.go

@@ -38,6 +38,7 @@ func Start(cfg *Config) error {
 
 	httpExpOpt := []otlpmetrichttp.Option{
 		otlpmetrichttp.WithEndpoint(cfg.Endpoint),
+		otlpmetrichttp.WithURLPath(cfg.HTTPPath),
 	}
 
 	if cfg.Insecure {

cmd/telemetrygen/internal/traces/traces.go

@@ -40,6 +40,7 @@ func Start(cfg *Config) error {
 
 	httpExpOpt := []otlptracehttp.Option{
 		otlptracehttp.WithEndpoint(cfg.Endpoint),
+		otlptracehttp.WithURLPath(cfg.HTTPPath),
 	}
 
 	if cfg.Insecure {