feat(spanner): add OpenTelemetry implementation

[harshachinta]

This PR adds support for OpenTelemetry Instrumentation for Traces and Metrics.

Customer applications should add dependency for Metrics SDK, Traces SDK and required exporters.

Metrics

1. By default, OpenTelemetry metrics are not enabled. To enable call spanner.EnableOpenTelemetryMetrics() in startup of your application.
2. Create MeterProvider and inject it via ClientConfig or register as Global.

client, err := spanner.NewClientWithConfig(ctx, <database-id>, spanner.ClientConfig{
	OpenTelemetryMeterProvider: meterProvider,
})

Tracing

1. By default, OpenTelemetry traces are not enabled. To enable set environment variable GOOGLE_API_GO_EXPERIMENTAL_TELEMETRY_PLATFORM_TRACING to opentelemetry
2. Create TracerProvider and register as Global

[olavloite]

It's a bit of a theoretical case, but shouldn't we split this into two separate things:

1. err != nil: return the error
2. !strings.HasPrefix(serverTiming, "gfet4t7; dur="): This should be considered the same as a missing GFE header (or malformed GFE header). Now we just return nil when that happens, is that intentional?

[harshachinta]

In case of malformed GFE header we return an error, not nil.
We are not counting malformed GFE as missing GFE because in this case we get a GFE header but it is not in required format.

[olavloite]

ack

note that this could still return err=nil, if the actual text is only a number without any other prefixes or suffixes. The reason is that strings.TrimPrefix returns the original string if the string did not have the prefix.

This for example prints 4:

fmt.Println(strconv.Atoi(strings.TrimPrefix("4", "World")))

[olavloite]

nit: could the variables here that never change be const?

[olavloite]

ack

note that this could still return err=nil, if the actual text is only a number without any other prefixes or suffixes. The reason is that strings.TrimPrefix returns the original string if the string did not have the prefix.

This for example prints 4:

fmt.Println(strconv.Atoi(strings.TrimPrefix("4", "World")))