Trailing slashes are stripped from trace endpoint URLs

[mjq]

Description

When setting an explicit OTLP traces endpoint URL (via otlptracehttp.WithEndpointURL or the OTEL_EXPORTER_OTLP_TRACES_ENDPOINT environment variable), any given trailing slash is stripped. This makes it impossible to export traces to an endpoint requiring a trailing slash. It also conflicts with the spec:

For the per-signal variables (OTEL_EXPORTER_OTLP_<signal>_ENDPOINT), the URL MUST be used as-is without any modification. The only exception is that if an URL contains no path part, the root path / MUST be used (see Example 2).

This stripping happens due to the use of path.Clean in otlpconfig.cleanPath. From the path.Clean docs:

The returned path ends in a slash only if it is the root "/".

Other signals appear to use path.Clean and might therefore have the same bug, but I've only tested traces.

Environment

• OS: macOS
• Architecture: arm64
• Go Version: 1.24.2
• opentelemetry-go version: v1.35.0, b4b461d (current main HEAD)

Steps To Reproduce

The following tests fail (with a URLPath of /somepath and /endpoint respectively).

diff --git a/exporters/otlp/otlptrace/otlptracehttp/internal/otlpconfig/options_test.go b/exporters/otlp/otlptrace/otlptracehttp/internal/otlpconfig/options_test.go
index 55a3ad96..16aadf6c 100644
--- a/exporters/otlp/otlptrace/otlptracehttp/internal/otlpconfig/options_test.go
+++ b/exporters/otlp/otlptrace/otlptracehttp/internal/otlpconfig/options_test.go
@@ -102,6 +102,15 @@ func TestConfigs(t *testing.T) {
                                assert.True(t, c.Traces.Insecure)
                        },
                },
+               {
+                       name: "Test With Endpoint URL With Trailing Slash",
+                       opts: []GenericOption{
+                               WithEndpointURL("http://someendpoint/somepath/"),
+                       },
+                       asserts: func(t *testing.T, c *Config, grpcOption bool) {
+                               assert.Equal(t, "/somepath/", c.Traces.URLPath)
+                       },
+               },
                {
                        name: "Test With Secure Endpoint URL",
                        opts: []GenericOption{
@@ -190,6 +199,17 @@ func TestConfigs(t *testing.T) {
                                }
                        },
                },
+               {
+                       name: "Test Environment Signal Specific Endpoint With Trailing Slash",
+                       env: map[string]string{
+                               "OTEL_EXPORTER_OTLP_TRACES_ENDPOINT": "http://env.traces/endpoint/",
+                       },
+                       asserts: func(t *testing.T, c *Config, grpcOption bool) {
+                               if !grpcOption {
+                                       assert.Equal(t, "/endpoint/", c.Traces.URLPath)
+                               }
+                       },
+               },
                {
                        name: "Test Mixed Environment and With Endpoint",
                        opts: []GenericOption{

Expected behavior

Trace endpoint URLs with trailing slashes should be used by the exporter without modification.