[HTTP] Report telemetry of the configured protocol (#211279)

## Summary

Starting with 9.0, we default to `http2` under certain conditions. We
want to report to telemetry how many deployments actively use `http2`.


### Checklist

- [x] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios

---------

Co-authored-by: kibanamachine <42973632+kibanamachine@users.noreply.github.com>

src/core/packages/usage-data/server-internal/src/core_usage_data_service.test.ts

@@ -307,6 +307,7 @@ describe('CoreUsageDataService', () => {
                 },
                 "keepaliveTimeout": 120000,
                 "maxPayloadInBytes": 1048576,
+                "protocol": "http1",
                 "requestId": Object {
                   "allowFromAnyIp": false,
                   "ipAllowlistConfigured": false,

src/core/packages/usage-data/server-internal/src/core_usage_data_service.ts

@@ -280,6 +280,7 @@ export class CoreUsageDataService
           rewriteBasePath: http.rewriteBasePath,
           keepaliveTimeout: http.keepaliveTimeout,
           socketTimeout: http.socketTimeout,
+          protocol: http.protocol,
           compression: {
             enabled: http.compression.enabled,
             referrerWhitelistConfigured: isConfigured.array(http.compression.referrerWhitelist),

src/core/packages/usage-data/server-mocks/src/core_usage_data_service.mock.ts

@@ -62,6 +62,7 @@ const createStartContractMock = () => {
             },
             keepaliveTimeout: 120000,
             maxPayloadInBytes: 1048576,
+            protocol: 'http1',
             requestId: {
               allowFromAnyIp: false,
               ipAllowlistConfigured: false,

src/core/packages/usage-data/server/src/core_usage_data.ts

@@ -91,6 +91,7 @@ export interface CoreConfigUsageData {
     rewriteBasePath: boolean;
     keepaliveTimeout: number;
     socketTimeout: number;
+    protocol: 'http1' | 'http2';
     compression: {
       enabled: boolean;
       referrerWhitelistConfigured: boolean;

src/platform/plugins/private/kibana_usage_collection/server/collectors/core/core_usage_collector.ts

@@ -151,6 +151,10 @@ export function getCoreUsageCollector(
             type: 'long',
             _meta: { description: 'Maximum payload size in bytes that is allowed.' },
           },
+          protocol: {
+            type: 'keyword',
+            _meta: { description: 'Protocol to serve the requests ("http1" | "http2")' },
+          },
           rewriteBasePath: {
             type: 'boolean',
             _meta: { description: 'Indicates if the base path should be rewritten.' },

src/platform/plugins/shared/telemetry/schema/oss_platform.json

@@ -8562,6 +8562,12 @@
                     "description": "Maximum payload size in bytes that is allowed."
                   }
                 },
+                "protocol": {
+                  "type": "keyword",
+                  "_meta": {
+                    "description": "Protocol to serve the requests (\"http1\" | \"http2\")"
+                  }
+                },
                 "rewriteBasePath": {
                   "type": "boolean",
                   "_meta": {