Make protobuf implementation optional (#949)

* Make google/protobuf an optional dependency
* Update the README with more information on installing protobuf implementation
* Add some extra documentation in the OTLP exporter library
* Update proto/otel/README.md

README.md

@@ -1 +1,21 @@
 # OpenTelemetry protobuf files
+
+## Protobuf Runtime library
+
+There exist two protobuf runtime libraries that offer the same set of APIs, allowing developers to choose the one that 
+best suits their needs.
+
+The first and easiest option is to install the Protobuf PHP Runtime Library through composer. This can be the easiest 
+way to get started quickly. Either run `composer require google/protobuf`, or update your `composer.json` as follows:
+
+```json
+"require": {
+  "google/protobuf": "^v3.3.0"
+}
+```
+
+Alternatively, and the recommended option for production is to install the Protobuf C extension for PHP. The extension
+makes both exporters _significantly_ more performant. This can be easily installed with the following command:
+```sh
+$ sudo pecl install protobuf
+```

composer.json

@@ -11,8 +11,7 @@
         }
     ],
     "require": {
-        "php": "^7.4 || ^8.0",
-        "google/protobuf": "^v3.3.0"
+        "php": "^7.4 || ^8.0"
     },
     "autoload": {
         "psr-4": {
@@ -22,6 +21,7 @@
     },
     "suggest": {
         "ext-protobuf": "For better performance, when dealing with the protobuf format",
+        "google/protobuf": "To get started quickly, install the native protobuf library.",
         "ext-grpc": "To use the gRPC based exporters"
     }
 }