fix: experimental metrics middleware should use the grpc message constructor name in emitted metrics instead of MiddlewareMessage

[anitarua]

Part of the CloudWatch metrics example work (see #970) in order to use requestType as a dimension on the dashboard graphs.

Also exports ExperimentalMetricsLoggingMiddleware at the top-level of the Node.js SDK and adds null and undefined checks for metrics messages (was running into issues with some middleware messages being undefined while working on #970).

Before this change:

[2023-10-30T22:05:13.777Z] INFO (Momento: ExperimentalMetricsLoggingMiddleware):
{
"numActiveRequestsAtStart":1,
"numActiveRequestsAtFinish":1,
"requestType":"MiddlewareMessage",
"status":0,"startTime":1698703513758,
"requestBodyTime":1698703513759,
"endTime":1698703513776,
"duration":18,
"requestSize":13,
"responseSize":9,
"connectionID":"1"
}

After:

[2023-10-30T22:17:17.087Z] INFO (Momento: ExperimentalMetricsLoggingMiddleware):
{
"numActiveRequestsAtStart":1,
"numActiveRequestsAtFinish":1,
"requestType":"_GetRequest",
"status":0,
"startTime":1698704237069,
"requestBodyTime":1698704237070,
"endTime":1698704237087,
"duration":18,
"requestSize":13,
"responseSize":9,
"connectionID":"3"
}

[anitarua]

@cprice404 Was able to reproduce this test failing locally: CacheClient › createWithEagerConnection returns a client even if it cannot connect

Seems like the if statements I put in were causing that. Will try moving the null check to the MiddlewareMessage.messageLength() function instead I guess based on the error message from the cloudwatch example work.

"TypeError: Cannot read properties of null (reading 'serializeBinary')",
"    at MiddlewareMessage.messageLength (/var/task/index.js:45889:34)",